feat(web): serve the interface at / — /v3 kept as a legacy alias

The user-visible URL no longer carries the interface version: the pages
blueprint is now registered un-prefixed (primary) AND at /v3 (second
registration, name='pages_v3_legacy'), so:

- http://<device>/ serves the interface directly (the old @app.route('/')
  redirect is removed — the blueprint's own index takes its place)
- every existing /v3/... bookmark and all the hardcoded /v3/partials/...
  fetches in templates/JS keep working verbatim through the alias mount —
  zero template/JS churn, zero broken links
- url_for('pages_v3.*') resolves against the primary registration, so all
  server-side redirects (captive portal detection endpoints) now emit
  un-prefixed URLs
- the AP-mode captive-portal allowlist learned the un-prefixed page paths
  (/setup, /partials/, /settings/, /plugin-ui/) so setup-mode requests
  don't redirect-loop
- /api/v3 and the templates/v3, static/v3 directories are deliberately
  untouched (internal, invisible to users; owner-confirmed scope)

Validation: dual registration mechanics tested against real Flask (test
client): /, /v3, /v3/ redirect, partials and /setup reachable on both
mounts, url_for yields un-prefixed paths; py_compile passes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEZK1P1Q1fu5pcuVrkrCFZ

web_interface/templates/v3/captive_setup.html

@@ -87,7 +87,7 @@ select:focus,input:focus{outline:none;border-color:#3b82f6;box-shadow:0 0 0 3px
 </div>
 
 <div class="footer">
-  <a href="/v3">Open Full Interface</a>
+  <a href="/">Open Full Interface</a>
 </div>
 
 <script>