feat(ui,api,db): implement notifications and custom templates with hand-drawn aesthetic

This commit introduces a significant update to both the frontend and backend, focusing on enhanced user engagement and a consistent visual identity.

Key changes include:

- **Frontend UI/UX Refactor**:
  - Implemented a "hand-drawn" aesthetic across the entire application using CSS overrides, custom SVG charts, and specific border/shadow styles to match the Excalidraw experience.
  - Added a new notification system in the Header to display user updates.
  - Enhanced the Template Picker with more variety and improved interaction models.
  - Added a "Presentation Mode" in the Editor.
  - Improved Dashboard visualizations with hand-drawn style sparklines and charts.
  - Added modal dialogs for creating drawings and templates with custom names.

- **Backend & API Enhancements**:
  - Implemented full CRUD support for custom templates, allowing users to save their drawings as reusable templates.
  - Added a notification service with endpoints to list, mark as read, and mark all as read.
  - Updated the API client to handle more robust JSON responses and error states.
  - Improved CORS/Origin validation in the HTTP middleware to handle proxy headers (`X-Forwarded-Host`, `X-Forwarded-Proto`) more reliably.

- **Database & Infrastructure**:
  - Added a new PostgreSQL migration for the `notifications` table.
  - Updated the data models in the workspace to support templates (including snapshot storage) and notifications.
  - Updated `.gitignore` to exclude graphify cache and AST files.

.gitignore

@@ -132,5 +132,12 @@ excalidraw-complete.exe
 # cf-kv worker output
 cf-kv/index.js
 
+# graphify cache and AST (keep output html/json)
+graphify-out/cache/
+graphify-out/.graphify_ast.json
+graphify-out/.graphify_detect.json
+graphify-out/.graphify_python
+graphify-out/.graphify_semantic.json
+
 # Keep dist/.keep for empty directory tracking in git
 !frontend/dist/.keep