webclient: Synapse reactions settings + service sessions toggle ...

- sidebar: service-sessions toggle (special=true fetch replaces the
  regular list, active search drops out), muted style + robot icon on
  special items in the list
- settings: Synapse reactions panel — enable switch, completion push
  preference (always / only failures / never), notification destination
  (app / app+Synapse / Synapse, Synapse options disabled while no
  source key), reaction instructions editor with save + edit history
  (author and reason per version)
- stores: synapseReactions store; sessions store fetches respect
  showSpecial; getSessions passes special param
- vitest for api/session store/panel; dist rebuilt

handbook alignment: profile.updated mirror, health contract, Synapse gateway ...

Handbook gaps closed (10-platform auth/health/notifications):

- webhooks: handle user.profile_updated — mirror the gnexus-auth profile
  into navi_users (name, contact fields, avatar_url — new column, boot
  migration); role/permissions keep their dedicated events
- auth: avatar_url now flows through the login upsert and the API-token
  resolution path
- /health: status is now the aggregate of the sub-checks (degraded embed
  or hive no longer reports plain ok); embed probe cached for 10 s; body
  grew the machine-readable checks{} map, legacy embed/hive payloads kept
- Synapse: s2s delivery gateway — POST /webhooks/synapse (both spellings),
  per-user delivery secrets (synapse_targets, Fernet-encrypted, shown
  once), verification via gnexus-synapse v0.1.2 client lib, idempotent
  event ids; deliveries are verified and logged for now — navi generates
  no outbound events by decision; web-push stays as the local browser
  channel
- webclient settings: Synapse deliveries panel (add/revoke targets)

Paginate session list loading

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>