Add notebook delete, fix button styling, rename Share button

- Add delete notebook handler with ownership check and CASCADE delete
- Rename "Create Share Link" to "Share"
- Fix action button heights: use inline-flex + align-items for
  consistent sizing across <a> and <button> elements

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

internal/webserver/server.go

@@ -112,6 +112,7 @@ func Start(cfg Config) (*http.Server, error) {
 		r.Get("/notebooks/{id}/pages/{num}/svg", ws.handlePageSVG)
 		r.Get("/notebooks/{id}/pages/{num}/jpg", ws.handlePageJPG)
 		r.Get("/notebooks/{id}/pdf", ws.handleNotebookPDF)
+		r.Post("/notebooks/{id}/delete", ws.handleDeleteNotebook)
 		r.Post("/notebooks/{id}/share", ws.handleCreateShare)
 		r.Post("/notebooks/{id}/share/revoke", ws.handleRevokeShare)
 		r.Get("/logout", ws.handleLogout)