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

`make frontend-viewport-test` builds `dist/`, serves it from the same
digest-pinned nginx image and nginx.conf the shipping container uses, and
drives a digest-pinned headless Chrome over CDP. Viewport widths are derived
from the app's own @media breakpoints rather than a list of phone models: each
breakpoint is tested one pixel below, on, and above, plus four anchor
viewports. Assertions are on computed layout — horizontal overflow, off-screen
elements, clipped text, 44x44 tap targets, host-row reflow — not screenshots,
and each check declares the minimum elements it must find so a stale selector
fails instead of passing blind against a page it is not measuring. Kept out of
`make check`: it needs Docker and takes minutes. Proven able to fail before
being trusted.

Model: opus-4-8

README.md

@@ -41,11 +41,25 @@ provide:
 - `script/fmt` — format all files (writes)
 - `script/fmt-check` — check formatting (read-only)
 - `script/check` — run test, lint, and fmt-check
+- `script/frontend-viewport-test` — responsive-layout verification of the built
+  frontend in a containerised headless Chrome (see
+  [test/viewport/README.md](test/viewport/README.md)). Not part of
+  `script/check`: it needs Docker and takes minutes.
 - `script/docker` — build the Docker image tagged via `script/projectname`
 - `script/cibuild` — CI entrypoint: plain `docker build .`
 - `script/precommit` — run by the git pre-commit hook; runs `script/check`
 - `script/install-precommit` — install the git pre-commit hook
 
+## Responsive layout
+
+The narrow-viewport layout lives in the `max-width: 768px` media block in
+`src/styles.css`. It is verified automatically by `make frontend-viewport-test`,
+which drives a digest-pinned headless Chrome against the built `dist/` and
+asserts on computed layout at widths derived from that CSS — one pixel either
+side of every breakpoint it declares, plus a 320px floor, a desktop baseline and
+two landscape sizes. See [test/viewport/README.md](test/viewport/README.md) for
+what it covers and what it genuinely cannot.
+
 ## Rationale
 
 When debugging network issues, it's useful to have a persistent at-a-glance view