feat: self-update checker with one-click update for WebUI + Agent

Shows a blue banner when the webui or hermes-agent git repos are behind
their upstream branches. One-click 'Update Now' button does stash, pull
--ff-only, stash pop, then reloads the page.

Backend (api/updates.py):
- _check_repo(): git fetch + rev-list count with 15s timeout
- check_for_updates(): 30-min server-side cache, thread-safe, skips
  Docker (no .git dir)
- apply_update(): stash (if dirty), pull --ff-only, pop, invalidate cache

Routes:
- GET /api/updates/check -- returns cached {webui, agent} with behind count
- POST /api/updates/apply -- {target: 'webui'|'agent'}

Frontend:
- Blue banner (matches reconnect-banner pattern) with 'Later' / 'Update Now'
- Non-blocking boot check via fire-and-forget .then(), once per tab session
- sessionStorage guards prevent re-checking and re-showing after dismiss

Settings:
- 'Check for updates' checkbox (default: on) -- when off, no git operations
- Removed 'Default Workspace' dropdown to keep settings panel compact

Performance:
- Server cache: git fetch at most 2x/hour regardless of client count
- sessionStorage: one check per browser tab session
- _check_in_progress flag prevents concurrent fetch storms
- Fire-and-forget: does NOT block the boot sequence

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/config.py

@@ -674,6 +674,7 @@ _SETTINGS_DEFAULTS = {
     'show_token_usage': False,  # show input/output token badge below assistant messages
     'show_cli_sessions': False,  # merge CLI sessions from state.db into the sidebar
     'sync_to_insights': False,  # mirror WebUI token usage to state.db for /insights
+    'check_for_updates': True,  # check if webui/agent repos are behind upstream
     'theme': 'dark',  # active UI theme name (no enum gate -- allows custom themes)
     'password_hash': None,  # SHA-256 hash; None = auth disabled
 }
@@ -694,7 +695,7 @@ _SETTINGS_ALLOWED_KEYS = set(_SETTINGS_DEFAULTS.keys()) - {'password_hash'}
 _SETTINGS_ENUM_VALUES = {
     'send_key': {'enter', 'ctrl+enter'},
 }
-_SETTINGS_BOOL_KEYS = {'show_token_usage', 'show_cli_sessions', 'sync_to_insights'}
+_SETTINGS_BOOL_KEYS = {'show_token_usage', 'show_cli_sessions', 'sync_to_insights', 'check_for_updates'}
 
 def save_settings(settings: dict) -> dict:
     """Save settings to disk. Returns the merged settings. Ignores unknown keys."""