feat: implement unified editor with content persistence and server-side upsert

- Replace dual update systems with single markdown-first editor architecture
- Add server-side upsert to eliminate 404 errors on PUT operations
- Fix content persistence race condition between preview and save operations
- Remove legacy updateElementContent system entirely
- Add comprehensive authentication with JWT scaffolding and dev mode
- Implement EditContext.updateOriginalContent() for proper baseline management
- Enable markdown formatting in all text elements (h1-h6, p, div, etc)
- Clean terminology: remove 'unified' references from codebase

Technical changes:
* core/editor.js: Remove legacy update system, unify content types as markdown
* ui/Editor.js: Add updateOriginalContent() method to fix save persistence
* ui/Previewer.js: Clean live preview system for all content types
* api/handlers.go: Implement UpsertContent for idempotent PUT operations
* auth/*: Complete authentication service with OAuth scaffolding
* db/queries/content.sql: Add upsert query with ON CONFLICT handling
* Schema: Remove type constraints, rely on server-side validation

Result: Clean content editing with persistent saves, no 404 errors, markdown support in all text elements

go.sum

@@ -7,6 +7,8 @@ github.com/frankban/quicktest v1.14.6 h1:7Xjx+VpznH+oBnejlPUj8oUpdxnVs4f8XU8WnHk
 github.com/frankban/quicktest v1.14.6/go.mod h1:4ptaffx2x8+WTWXmUCuVU6aPUX1/Mz7zb5vbUoiM6w0=
 github.com/fsnotify/fsnotify v1.7.0 h1:8JEhPFa5W2WU7YfeZzPNqzMP6Lwt7L2715Ggo0nosvA=
 github.com/fsnotify/fsnotify v1.7.0/go.mod h1:40Bi/Hjc2AVfZrqy+aj+yEI+/bRxZnMJyTJwOpGvigM=
+github.com/golang-jwt/jwt/v5 v5.3.0 h1:pv4AsKCKKZuqlgs5sUmn4x8UlGa0kEVt/puTpKx9vvo=
+github.com/golang-jwt/jwt/v5 v5.3.0/go.mod h1:fxCRLWMO43lRc8nhHWY6LGqRcf+1gQWArsqaEUEa5bE=
 github.com/google/go-cmp v0.5.9 h1:O2Tfq5qg4qc4AmwVlvv0oLiVAGB7enBSJ2x2DqQFi38=
 github.com/google/go-cmp v0.5.9/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
 github.com/gorilla/mux v1.8.1 h1:TuBL49tXwgrFYWhqrNgrUNEY92u81SPhu7sTdzQEiWY=