Feat default surface routing + fix startup hang (#4)

Default surface routing (v0.2.1):
- `/` redirects to configurable default surface with fallback chain:
  configured → first enabled extension surface → /admin
- Auto-sets default_surface on first extension surface install
- Admin settings UI: Default Surface dropdown
- disabledRedirect() and extension surface disabled redirect now go to
  /admin instead of / to prevent redirect loops

Fix Gin route param conflict causing backend startup hang:
- Team package settings routes used `:pkgId` while sibling routes used
  `:id` — Gin's radix tree entered infinite loop on conflicting param
  names. Unified to `:id`.

Docker entrypoint hardening:
- Health check timeout 10s → 60s
- Stale backend process cleanup on restart
- Crash detection (exit early if backend dies)

Roadmap direction updates:
- v0.3.0: Notes surface (Obsidian-style) replaces Editor surface
- v0.4.0: MVP (was v0.5.0) — chat moved to post-MVP
- No built-in auto-install — explicit install only

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

server/pages/pages.go

@@ -391,7 +391,7 @@ func (e *Engine) RenderExtensionSurface() gin.HandlerFunc {
 			return
 		}
 		if !sr.Enabled {
-			c.Redirect(http.StatusTemporaryRedirect, e.cfg.BasePath+"/")
+			c.Redirect(http.StatusTemporaryRedirect, e.cfg.BasePath+"/admin")
 			return
 		}
 		if sr.Source == "core" {