Add rendering routes and share UI to web server

The web UI was linking to /v1/ REST API paths that aren't served
through nginx. Added SVG/JPG/PDF rendering and share link endpoints
directly to the web server so everything works through port 443.

- Add render.go with SVG, JPG, PDF handlers for auth and share paths
- Register render routes and share management routes in web server
- Update template links from /v1/... to /notebooks/... paths
- Add share link creation, display, and revocation to notebook view

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

web/templates/notebook.html

@@ -4,7 +4,28 @@
 <h1>{{.Title}}</h1>
 <div class="actions">
     <a href="{{.PDFLink}}" class="btn">Download PDF</a>
+    {{if not .Shared}}
+    <form method="POST" action="/notebooks/{{.ID}}/share" style="display:inline;">
+        <button type="submit" class="btn">Create Share Link</button>
+    </form>
+    {{end}}
 </div>
+
+{{if .ShareLinks}}
+<div style="margin: 1rem 0; padding: 0.75rem; border: 1px solid #ccc; border-radius: 4px;">
+    <strong>Share Links</strong>
+    {{range .ShareLinks}}
+    <div style="display: flex; align-items: center; gap: 0.5rem; margin-top: 0.5rem;">
+        <code style="font-size: 0.85rem; flex: 1;"><a href="{{.URL}}">{{.URL}}</a></code>
+        <form method="POST" action="/notebooks/{{$.ID}}/share/revoke" style="margin:0;">
+            <input type="hidden" name="token" value="{{.Token}}">
+            <button type="submit" class="btn" style="font-size: 0.8rem; padding: 0.25rem 0.5rem;">Revoke</button>
+        </form>
+    </div>
+    {{end}}
+</div>
+{{end}}
+
 <div class="grid">
     {{range .Pages}}
     <div>