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/notebook.html

@@ -0,0 +1,20 @@
+{{define "title"}}{{.Title}} — Engineering Pad{{end}}
+
+{{define "content"}}
+<h1>{{.Title}}</h1>
+<div class="actions">
+    <a href="{{.PDFLink}}" class="btn">Download PDF</a>
+</div>
+<div class="grid">
+    {{range .Pages}}
+    <div>
+        <a href="{{.ViewLink}}">
+            <div class="page-thumb">
+                <img src="{{.SVGLink}}" alt="Page {{.Number}}">
+            </div>
+        </a>
+        <div style="text-align:center; margin-top:0.25rem;">Page {{.Number}}</div>
+    </div>
+    {{end}}
+</div>
+{{end}}