Clear offline data on logout, test offline plan and recipe reads, drop Pillow

- POST /auth/logout answers with Clear-Site-Data: "storage", "cache", which
  removes the verified-session marker, the service worker and its caches, so
  an offline visit after logout cannot reopen household data. The 401/403
  clearing path is unchanged.
- The service worker no longer answers network-only requests; it leaves them
  to the browser. Chrome stalls a Clear-Site-Data navigation that the worker
  answered, so the logout POST must bypass it.
- New tests send plan and recipe reads through the real API client and the
  real network-first strategy online, then serve them from the cache offline.
- The icon generator is now Go on golang.org/x/image (approved) instead of
  Python on Pillow; the icons are regenerated with it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/ARCHITECTURE.md

@@ -208,6 +208,9 @@ Runs are recorded in `schedule_runs` and executed through `jobs`.
   hard-coded hex. Self-hosted Bagel Fat One and Figtree.
 - Tests: Vitest, Testing Library, jsdom.
 - Offline cache for the current plan, shopping list and recipes.
+- Logout (`POST /auth/logout`) answers with `Clear-Site-Data: "storage",
+  "cache"`, wiping the offline marker, the service worker and its caches; the
+  worker leaves that request to the browser, as it does every network-only one.
 
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