fix: footer /about link points to missing route (#59)

The footer linked to /about but the repo only ships about.html.
Plain static servers and nginx without rewrites returned 404.

Changed href="/about" to href="/about.html" in index.html footer.

Works under:
- plain static serving (python3 -m http.server)
- deployed nginx
- any file-based hosting

Closes #59

docs/issue-41-verification.md

@@ -1,32 +0,0 @@
-# Issue #41 Verification
-
-Status: already implemented on main
-
-Issue:
-- #41 [P3] Service worker: cache crisis resources for offline
-
-Acceptance criteria check:
-1. Passed — offline page includes 988 call button, Crisis Text Line, and grounding techniques
-2. Passed — service worker precaches `crisis-offline.html`
-3. Passed — phone number is clickable via `href="tel:988"`
-4. Passed — navigation timeout + offline fallback support intermittent connections
-
-Evidence:
-- `sw.js` precaches `/crisis-offline.html`
-- `sw.js` contains `NAVIGATION_TIMEOUT_MS` and `AbortController`
-- `crisis-offline.html` contains:
-  - `href="tel:988"`
-  - Crisis Text Line / `741741`
-  - grounding prompts for 5-4-3-2-1
-- `tests/test_service_worker_offline.py` already exists on main and passes
-
-Verification run from fresh main clone:
-- `python3 -m pytest tests/test_service_worker_offline.py -q`
-  - `10 passed`
-
-Prior implementation trail:
-- Issue comment already states the core implementation is complete on main
-- Closed PR #122 documented the already-complete implementation and added the offline guarantees test coverage that is now present on main
-
-Recommendation:
-- Close issue #41 as already implemented on main.