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/page.html

@@ -0,0 +1,13 @@
+{{define "title"}}Page {{.PageNumber}} — {{.NotebookTitle}} — Engineering Pad{{end}}
+
+{{define "content"}}
+<h1>{{.NotebookTitle}} — Page {{.PageNumber}}</h1>
+<div class="actions">
+    <a href="{{.BackLink}}" class="btn">Back to notebook</a>
+    <a href="{{.JPGLink}}" class="btn">Download JPG</a>
+    <a href="{{.PDFLink}}" class="btn">Download PDF</a>
+</div>
+<div style="border: 1px solid #ccc; background: #fff;">
+    <img src="{{.SVGLink}}" alt="Page {{.PageNumber}}" style="width: 100%; height: auto;">
+</div>
+{{end}}