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.

workspace/models.go

@@ -143,6 +143,14 @@ type Template struct {
 	PreviewURL   *string        `json:"preview_url,omitempty"`
 }
 
+type CreateTemplateRequest struct {
+	TeamID      string          `json:"team_id"`
+	Name        string          `json:"name"`
+	Description string          `json:"description"`
+	Snapshot    json.RawMessage `json:"snapshot"`
+	Metadata    map[string]any  `json:"metadata"`
+}
+
 type ActivityEvent struct {
 	ID           string         `json:"id"`
 	ActorUserID  *string        `json:"actor_user_id"`
@@ -204,6 +212,19 @@ type LinkReference struct {
 	CreatedAt          time.Time `json:"created_at"`
 }
 
+type Notification struct {
+	ID           string         `json:"id"`
+	UserID       string         `json:"user_id"`
+	Type         string         `json:"type"`
+	Title        string         `json:"title"`
+	Description  string         `json:"description"`
+	ResourceType string         `json:"resource_type,omitempty"`
+	ResourceID   string         `json:"resource_id,omitempty"`
+	Read         bool           `json:"read"`
+	MetadataJSON map[string]any `json:"metadata_json"`
+	CreatedAt    time.Time      `json:"created_at"`
+}
+
 type WorkspaceStats struct {
 	Teams        int   `json:"teams"`
 	Members      int   `json:"members"`