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

TODO.md

@@ -44,6 +44,14 @@ files, so merging it also closes most compliance gaps.
   ignored repo-wide, not just under `backend/`. Excluding `.git` from
   `.dockerignore` stays deferred: both images read git metadata at build time
   (`COPY .git` in `Dockerfile.backend`, `git rev-parse` in `vite.config.js`)
+- 2026-08-09: automated responsive-layout harness
+  (`make frontend-viewport-test`): digest-pinned headless Chrome driven over CDP
+  against the built `dist/`, viewport widths derived from the breakpoints in
+  `src/styles.css` ([#13](https://git.eeqj.de/sneak/netwatch/issues/13)). Every
+  check carries a presence guard so none of them can pass against a page it is
+  not actually measuring. Found two real layout defects, filed as
+  [#42](https://git.eeqj.de/sneak/netwatch/issues/42) and
+  [#43](https://git.eeqj.de/sneak/netwatch/issues/43)
 - 2026-07-07 Adopted scripts-to-rule-them-all: `script/` entrypoints, Makefile
   shims, README Entrypoints section
 - 2026-02-27: backend with buffered zstd-compressed report storage; CI workflow
@@ -61,6 +69,9 @@ files, so merging it also closes most compliance gaps.
 
 # Future Steps
 
+- Wire `script/frontend-viewport-test` into CI as its own step (deliberately not
+  part of `make check` today; the decision has real CI-runtime cost and is
+  tracked separately)
 - Compliance top-up as one small commit: add .editorconfig and add the hooks
   target to the Makefile
 - After merge, confirm .gitea/workflows/check.yml is on main and CI is green