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

package.json

@@ -14,6 +14,7 @@
         "autoprefixer": "^10.4.23",
         "postcss": "^8.5.6",
         "prettier": "^3.8.1",
+        "puppeteer-core": "25.5.0",
         "tailwindcss": "^4.1.18",
         "vite": "^7.3.1"
     }