test: automated responsive-layout harness (closes #13)

Verifies the mobile layout from #5 with a real browser engine instead of
by hand on a phone. make frontend-viewport-test builds dist/, serves it
from the same digest-pinned nginx image and the same nginx.conf the
shipping container uses, and drives a digest-pinned headless Chrome
against it over CDP.

Viewport widths are derived from the app's own CSS rather than from a
list of phone models: the @media conditions in src/styles.css and any
Tailwind responsive prefixes in the markup are parsed, and each
breakpoint is tested one pixel below, exactly on, and one pixel above.
max-width: 768px matches at 768, and a generic 375px test sails past
that boundary entirely. Four anchor viewports are added with stated
reasons: a 320px floor, a desktop baseline, and two phone-landscape
sizes straddling the breakpoint.

Assertions are on computed layout, not screenshots: horizontal overflow,
elements past the viewport edge, clipped text (deliberate ellipsis
truncation excluded), 44x44 minimum tap targets, and genuine reflow of
the host rows checked on both flex-direction and geometry. Probing and
gateway detection are asserted to still run at narrow widths, since the
early-return mobile path rejected in #8 is what would silently regress.
Screenshots are written to tmp/viewport/ as artifacts alongside the
results, not as the evidence.

puppeteer-core rather than playwright: it is the one variant of either
that never downloads or bundles a browser, so the browser stays a
digest-pinned image and the npm side is pinned by yarn.lock integrity.

The browser container runs on an --internal docker network with no route
off the host; the harness answers the app's latency probes itself from a
fixed delay table so the rows render a realistic spread of value widths.

Kept out of make check: it needs Docker and takes minutes, where make
test has to stay under 20 seconds.

The harness was observed failing before being trusted, twice: a planted
900px fixed-width element in a host row, and the mobile reflow rule
neutered. Both reverted.

Against the current layout it reports two real defects, filed as #42
(horizontal overflow at 320px) and #43 (tap targets below 44x44).

test/viewport/viewports.js

@@ -0,0 +1,185 @@
+// Viewport derivation for the responsive-layout harness.
+//
+// The widths tested are read out of the CSS the application actually
+// ships, not taken from a list of popular phone models. A generic 375px
+// "phone" test sails straight past an off-by-one error at a media query
+// boundary, which is the classic way a responsive layout breaks, so
+// every breakpoint found in the sources is probed three times: one pixel
+// below it, exactly on it, and one pixel above it.
+//
+// Nothing here hardcodes 768. If someone adds a second media block or
+// starts using Tailwind responsive prefixes, that breakpoint starts
+// being covered without this file being edited.
+
+import { readFileSync } from "node:fs";
+import { join } from "node:path";
+
+// Tailwind CSS v4 default breakpoints, in rem. The app currently uses
+// none of these prefixes, so the whole table is inert until someone
+// writes an `md:`-prefixed utility class.
+const TAILWIND_BREAKPOINT_REM = {
+    sm: 40,
+    md: 48,
+    lg: 64,
+    xl: 80,
+    "2xl": 96,
+};
+
+// The app does not override the root font size, so rem and em in media
+// queries resolve against the browser default.
+const ROOT_FONT_SIZE_PX = 16;
+
+// Extract every min-width / max-width condition from the @media blocks in
+// a stylesheet. Returns e.g. [{ type: "max", px: 768, source: "..." }].
+export function mediaConditionsFromCss(css, source) {
+    const conditions = [];
+    for (const block of css.matchAll(/@media([^{]+)\{/g)) {
+        const features = block[1].matchAll(
+            /\(\s*(min|max)-width\s*:\s*([\d.]+)(px|rem|em)\s*\)/g,
+        );
+        for (const feature of features) {
+            const scale = feature[3] === "px" ? 1 : ROOT_FONT_SIZE_PX;
+            conditions.push({
+                type: feature[1],
+                px: Math.round(Number(feature[2]) * scale),
+                source,
+            });
+        }
+    }
+    return conditions;
+}
+
+// Extract the breakpoints implied by Tailwind responsive prefixes used in
+// markup. A prefix only counts when it opens a utility class, so `text-sm`
+// does not masquerade as the `sm:` breakpoint.
+export function mediaConditionsFromMarkup(sources) {
+    const conditions = [];
+    for (const { path, text } of sources) {
+        for (const [name, rem] of Object.entries(TAILWIND_BREAKPOINT_REM)) {
+            const used = new RegExp(
+                `(^|["'\\s])${name}:[a-z0-9[\\](),_./%-]+`,
+                "m",
+            ).test(text);
+            if (used) {
+                conditions.push({
+                    type: "min",
+                    px: rem * ROOT_FONT_SIZE_PX,
+                    source: path,
+                });
+            }
+        }
+    }
+    return conditions;
+}
+
+// Whether a given width should be rendering the app's narrow (stacked)
+// layout. The app keeps all of its narrow-viewport rules inside
+// `max-width` blocks, so a width is narrow exactly when one of those
+// blocks matches. Note that `max-width: 768px` matches *at* 768: getting
+// this inclusive boundary wrong in either direction is precisely what the
+// three-widths-per-breakpoint sweep exists to catch.
+export function expectsStackedLayout(width, conditions) {
+    return conditions.some((c) => c.type === "max" && width <= c.px);
+}
+
+// Viewports that are not derived from a breakpoint. Each one is here for
+// a stated reason; none of them is a stand-in for "a phone".
+const ANCHOR_VIEWPORTS = [
+    {
+        name: "floor-portrait",
+        width: 320,
+        height: 568,
+        deviceScaleFactor: 2,
+        touch: true,
+        why: "320px is the narrowest viewport still in mainstream use; nothing has to work below it",
+    },
+    {
+        name: "phone-landscape-narrow",
+        width: 667,
+        height: 375,
+        deviceScaleFactor: 2,
+        touch: true,
+        why: "phone rotated to landscape, still inside the narrow layout",
+    },
+    {
+        name: "phone-landscape-wide",
+        width: 844,
+        height: 390,
+        deviceScaleFactor: 3,
+        touch: true,
+        why: "large phone rotated to landscape: crosses into the wide layout while still being a touch device",
+    },
+    {
+        name: "desktop",
+        width: 1280,
+        height: 800,
+        deviceScaleFactor: 1,
+        touch: false,
+        why: "desktop baseline",
+    },
+];
+
+export function deriveViewports(root) {
+    const conditions = [
+        ...mediaConditionsFromCss(
+            readFileSync(join(root, "src/styles.css"), "utf8"),
+            "src/styles.css",
+        ),
+        ...mediaConditionsFromMarkup([
+            {
+                path: "src/main.js",
+                text: readFileSync(join(root, "src/main.js"), "utf8"),
+            },
+            {
+                path: "index.html",
+                text: readFileSync(join(root, "index.html"), "utf8"),
+            },
+        ]),
+    ];
+
+    if (conditions.length === 0) {
+        throw new Error(
+            "no responsive breakpoints found in src/styles.css, src/main.js or " +
+                "index.html — either the responsive layout was deleted or this " +
+                "derivation has stopped matching the sources",
+        );
+    }
+
+    const viewports = new Map();
+    const add = (viewport) => {
+        const key = `${viewport.width}x${viewport.height}`;
+        if (!viewports.has(key)) viewports.set(key, viewport);
+    };
+
+    for (const condition of conditions) {
+        for (const [offset, label] of [
+            [-1, "below"],
+            [0, "at"],
+            [+1, "above"],
+        ]) {
+            const width = condition.px + offset;
+            add({
+                name: `${condition.type}-width-${condition.px}-${label}`,
+                width,
+                // Tall enough that the whole app is laid out in one column
+                // without the viewport height influencing wrapping.
+                height: 1024,
+                deviceScaleFactor: 2,
+                touch: true,
+                why: `${offset === 0 ? "exactly on" : `1px ${label}`} the ${condition.type}-width: ${condition.px}px breakpoint declared in ${condition.source}`,
+            });
+        }
+    }
+
+    for (const anchor of ANCHOR_VIEWPORTS) add(anchor);
+
+    return {
+        conditions,
+        viewports: [...viewports.values()]
+            .map((viewport) => ({
+                ...viewport,
+                expectStacked: expectsStackedLayout(viewport.width, conditions),
+            }))
+            .sort((a, b) => a.width - b.width || a.height - b.height),
+    };
+}