Remove erroneous vue-router from webclient ...

vue-router was accidentally left over from an earlier attempt to embed
admin features in the main client. It broke hash-based session routing
and rendered app-main empty. Revert to direct component rendering in
App.vue with native hashchange handling.

- Remove vue-router dependency and src/router/index.js
- Revert main.js to createApp without router
- App.vue: render WelcomeScreen/ChatArea directly, restore hashchange handler
- chat.js: revert hash format to raw session id
- api/index.js: keep 401 → authStore.user = null (login overlay fix)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Fix blank session rendering and 401 auth handling ...

- Install missing vue-router dependency (was imported but not in node_modules,
  causing app init failure and empty app-main)
- Add :sessionId? route param and HomeView watcher for hash-based session routing
- Update hash format to '#/' for Vue Router compatibility
- Add 401 handling in api/index.js to invalidate auth and show login overlay
- Add error handling in loadSession and WelcomeScreen start to prevent stuck UI

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Add webclient unit tests (Vitest) — 47 tests ...

- Vitest + @vue/test-utils + happy-dom setup
- api/index.test.js: 8 tests for fetch wrapper, verbs, errors, FormData
- stores/chat.test.js: 23 tests for buildMessageList, WS handlers, load/clear
- stores/sessions.test.js: 6 tests for fetch, create, delete, pin, preview
- stores/profiles.test.js: 3 tests for fetch, auto-select, lookup
- composables/useWebSocket.test.js: 7 tests with MockWebSocket
- Export buildMessageList from chat store so tests can verify message parsing
- Update docs/testing.md with webclient stack, layout, and commands

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Migrate to Vue webclient; rename old client to old_webclient ...

- client/ → old_webclient/ (vanilla JS client preserved as reference)
- webclient/ — new Vue 3 + Pinia webclient (source + dist build)
- vite.config.js: outDir changed to webclient/dist/
- main.py: serve /assets and /images from webclient/dist/,
  index.html from webclient/dist/index.html
- .gitignore: exclude webclient/node_modules/, include webclient/dist/

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>