Implement Phase 8: Web UI with htmx templates

- HTML templates: layout, login, notebook list, notebook view, page viewer
- Web server with chi router, embedded templates via //go:embed
- Login/logout flow with session cookies
- Notebook list, page grid with SVG thumbnails, page viewer
- Share link views (same templates, no auth chrome)
- Server command wired to start gRPC + REST + web servers concurrently
- Graceful shutdown on SIGINT/SIGTERM

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

web/templates/notebooks.html

@@ -0,0 +1,17 @@
+{{define "title"}}Notebooks — Engineering Pad{{end}}
+
+{{define "nav-right"}}<a href="/logout">Logout</a>{{end}}
+
+{{define "content"}}
+<h1>Notebooks</h1>
+{{range .Notebooks}}
+<a class="card-link" href="/notebooks/{{.ID}}">
+    <div class="card">
+        <strong>{{.Title}}</strong>
+        <br><small>{{.PageSize}} — {{.PageCount}} pages — synced {{.SyncedAt}}</small>
+    </div>
+</a>
+{{else}}
+<p>No notebooks synced yet.</p>
+{{end}}
+{{end}}