feat(router): add collection modal routes for HTMX

Add three new frontend routes to support HTMX-powered modal dialogs:

1. GET /collections/create-modal
   - Renders empty collection creation modal
   - Uses CollectionModal template with empty CollectionData

2. GET /collections/:id/edit-modal
   - Fetches collection by ID from database
   - Pre-populates modal with existing collection data
   - Returns 400 for invalid UUID, 404 if collection not found

3. GET /collections/restore-modal
   - Renders system collection restoration modal
   - Allows users to restore deleted system collections

Route registration order:
- /collections/:id/edit-modal must be registered before /collections/:id
  to avoid path conflicts in Echo's router

These routes enable the collections page to load modals dynamically via
HTMX (hx-get) instead of embedding modal HTML in the base page.

internal/router/frontend.go

@@ -238,6 +238,63 @@ func registerFrontendRoutes(cfg *Config) {
 		return c.HTML(http.StatusOK, buf.String())
 	})
 
+	// Collection create modal
+	frontendProtected.GET("/collections/create-modal", func(c echo.Context) error {
+		var buf bytes.Buffer
+		err := templates.CollectionModal(templates.CollectionData{}).Render(c.Request().Context(), &buf)
+		if err != nil {
+			return err
+		}
+		return c.HTML(http.StatusOK, buf.String())
+	})
+
+	// Collection edit modal
+	frontendProtected.GET("/collections/:id/edit-modal", func(c echo.Context) error {
+		// Parse collection ID
+		collectionID := c.Param("id")
+		collUUID, err := uuid.Parse(collectionID)
+		if err != nil {
+			return c.HTML(http.StatusBadRequest, "<div>Invalid collection ID</div>")
+		}
+
+		// Fetch collection data
+		collection, err := cfg.Queries.GetCollection(c.Request().Context(), pgtype.UUID{Bytes: collUUID, Valid: true})
+		if err != nil {
+			return c.HTML(http.StatusNotFound, "<div>Collection not found</div>")
+		}
+
+		// Convert to template type
+		colData := templates.CollectionData{
+			ID:          uuid.UUID(collection.ID.Bytes).String(),
+			Name:        collection.Name,
+			Description: getText(collection.Description),
+			Color:       getText(collection.Color),
+			Icon:        getText(collection.Icon),
+		}
+
+		// Ensure default color if empty
+		if colData.Color == "" {
+			colData.Color = "blue"
+		}
+
+		var buf bytes.Buffer
+		err = templates.CollectionModal(colData).Render(c.Request().Context(), &buf)
+		if err != nil {
+			return err
+		}
+		return c.HTML(http.StatusOK, buf.String())
+	})
+
+	// Restore system collection modal
+	frontendProtected.GET("/collections/restore-modal", func(c echo.Context) error {
+		var buf bytes.Buffer
+		err := templates.RestoreSystemCollectionModal().Render(c.Request().Context(), &buf)
+		if err != nil {
+			return err
+		}
+		return c.HTML(http.StatusOK, buf.String())
+	})
+
 	// Collection detail page (works for both system and user collections)
 	frontendProtected.GET("/collections/:id", func(c echo.Context) error {
 		user, err := getTemplateUserWithTheme(c, cfg)