feat(maintenance): improve store with pending removals + ADR-0006 compliance

- Add pendingRemovals tracking and isEventRemoving() to maintenance store for better in-flight UX in EventsTable.
- Strongly type maintenance API client and store (replace unknown[]).
- Migrate selectedTask/taskLogs to dedicated lib/stores/selectedTask.svelte.ts (proper rune store with separate subscribers).
- Remove legacy central stores.svelte.ts and related dead code (src/pages/, old tests) to comply with ADR-0006 (global stores must be individual files in lib/stores/).
- Update mocks, components, models and tests accordingly.
- Fix related bugs (subscriber decoupling, test mocks).
- Add dedicated test_selectedTask.ts.
- Minor cleanups: semantic tokens, @RATIONALE for model-first, contracts update.
- All key tests (maintenance, selectedTask, store) pass.

Refs: ADR-0006, specs/031-maintenance-banner

frontend/tests/maintenance.test.ts

@@ -14,6 +14,7 @@ const { mockStoreState, mockStore } = vi.hoisted(() => {
         dashboardBanners: [],
         isLoading: false,
         error: null,
+        pendingRemovals: {},
     };
 
     const store = {
@@ -23,6 +24,8 @@ const { mockStoreState, mockStore } = vi.hoisted(() => {
         get dashboardBanners() { return sharedState.dashboardBanners; },
         get isLoading() { return sharedState.isLoading; },
         get error() { return sharedState.error; },
+        get pendingRemovals() { return sharedState.pendingRemovals; },
+        isEventRemoving: vi.fn(() => false),
         loadEvents: vi.fn(),
         loadSettings: vi.fn(),
         loadDashboardBanners: vi.fn(),
@@ -44,6 +47,7 @@ function resetMockStore() {
     mockStoreState.dashboardBanners = [];
     mockStoreState.isLoading = false;
     mockStoreState.error = null;
+    mockStoreState.pendingRemovals = {};
 }
 
 // Mock toasts