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>

static/style.css

@@ -145,6 +145,13 @@
   .reconnect-banner.visible{display:flex;}
   .reconnect-btn{padding:5px 12px;border-radius:7px;font-size:12px;font-weight:600;background:rgba(201,168,76,0.15);border:1px solid rgba(201,168,76,0.4);color:var(--gold);cursor:pointer;}
   .reconnect-btn:hover{background:rgba(201,168,76,0.25);}
+  /* ── Update banner ── */
+  .update-banner{display:none;background:var(--surface);border:1px solid rgba(124,185,255,0.4);border-radius:10px;padding:10px 16px;margin:10px auto;max-width:780px;font-size:13px;color:var(--blue);align-items:center;justify-content:space-between;gap:12px;}
+  .update-banner.visible{display:flex;}
+  .update-btn{padding:5px 12px;border-radius:7px;font-size:12px;font-weight:600;background:rgba(124,185,255,0.1);border:1px solid rgba(124,185,255,0.3);color:var(--blue);cursor:pointer;transition:background .15s;}
+  .update-btn:hover{background:rgba(124,185,255,0.2);}
+  .update-primary{background:rgba(124,185,255,0.2);border-color:rgba(124,185,255,0.5);}
+  .update-btn:disabled{opacity:0.5;cursor:not-allowed;}
   /* ── Approval card ── */
   .approval-card{display:none;max-width:780px;margin:0 auto 0;padding:0 20px 12px;}
   .approval-card.visible{display:block;}