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/static/htmx.min.js

@@ -0,0 +1,3 @@
+// htmx placeholder — download from https://unpkg.com/htmx.org
+// This is a minimal stub for initial development.
+console.log("htmx stub loaded");