feat(dashboard): add HTMX form data binding and redirect to RestoreSystemCollection

Update RestoreSystemCollection handler to support form-encoded requests from HTMX:

- Add 'form' struct tags to CollectionName and ResetType fields to enable binding
  from both JSON payloads and form submissions (required for HTMX compatibility)
- Add conditional HTMX redirect handling that sets HX-Redirect header when
  the request originates from HTMX, directing users to /collections after
  successful restoration

This change enables the system collection restore functionality to work seamlessly
with HTMX-based modal forms, improving the user experience by providing proper
navigation after the restore operation completes without requiring JavaScript
redirect logic.

internal/handlers/dashboard.go

@@ -111,8 +111,8 @@ func (h *DashboardHandler) RestoreSystemCollection(c echo.Context) error {
 	userUUID := uuid.UUID(user.ID.Bytes)
 
 	var req struct {
-		CollectionName string `json:"collection_name"`
-		ResetType      string `json:"reset_type"` // "full" or "keep_books"
+		CollectionName string `form:"collection_name" json:"collection_name"`
+		ResetType      string `form:"reset_type" json:"reset_type"`
 	}
 
 	if err := c.Bind(&req); err != nil {
@@ -146,6 +146,11 @@ func (h *DashboardHandler) RestoreSystemCollection(c echo.Context) error {
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "Failed to restore system collection"})
 	}
 
+	// Add redirect header for HTMX requests
+	if c.Request().Header.Get("HX-Request") == "true" {
+		c.Response().Header().Set("HX-Redirect", "/collections")
+	}
+
 	return c.JSON(http.StatusOK, map[string]string{"message": "System collection restored to defaults"})
 }