feat: Phase 3 - filter bar, keyboard navigation, notifications, comments

- FilterBar component with text search, label chips, due date and priority dropdowns
- "/" keyboard shortcut and toolbar button to toggle filter bar
- Keyboard card navigation with J/K/H/L keys, Enter to open, Escape to clear
- Focus ring on keyboard-selected cards with auto-scroll
- Desktop notifications for due/overdue cards via tauri-plugin-notification
- CommentsSection component with add/delete and relative timestamps
- Filtered card count display in column headers

src/types/settings.ts

@@ -8,6 +8,8 @@ export interface WindowState {
   maximized: boolean;
 }
 
+export type BoardSortOrder = "manual" | "title" | "created" | "updated";
+
 export interface AppSettings {
   theme: "light" | "dark" | "system";
   dataDirectory: string | null;
@@ -17,4 +19,7 @@ export interface AppSettings {
   density: "compact" | "comfortable" | "spacious";
   defaultColumnWidth: ColumnWidth;
   windowState: WindowState | null;
+  boardSortOrder: BoardSortOrder;
+  boardManualOrder: string[];
+  lastNotificationCheck: string | null;
 }