feat: @agor/stores package (3 stores) + 58 BackendAdapter tests

@agor/stores:
- theme.svelte.ts, notifications.svelte.ts, health.svelte.ts extracted
- Original files replaced with re-exports (zero consumer changes needed)
- pnpm workspace + Vite/tsconfig aliases configured

BackendAdapter tests (58 new):
- backend-adapter.test.ts: 9 tests (lifecycle, singleton, testing seam)
- tauri-adapter.test.ts: 28 tests (invoke mapping, command names, params)
- electrobun-adapter.test.ts: 21 tests (RPC names, capabilities, stubs)

Total: 523 tests passing (was 465, +58)

ui-electrobun/src/mainview/SettingsDrawer.svelte

@@ -8,6 +8,7 @@
   import MarketplaceTab           from './settings/MarketplaceTab.svelte';
   import KeyboardSettings         from './settings/KeyboardSettings.svelte';
   import RemoteMachinesSettings   from './settings/RemoteMachinesSettings.svelte';
+  import DiagnosticsTab           from './settings/DiagnosticsTab.svelte';
 
   interface Props {
     open: boolean;
@@ -16,7 +17,7 @@
 
   let { open, onClose }: Props = $props();
 
-  type CategoryId = 'appearance' | 'agents' | 'security' | 'projects' | 'orchestration' | 'machines' | 'advanced' | 'marketplace' | 'keyboard';
+  type CategoryId = 'appearance' | 'agents' | 'security' | 'projects' | 'orchestration' | 'machines' | 'advanced' | 'marketplace' | 'keyboard' | 'diagnostics';
 
   interface Category {
     id: CategoryId;
@@ -34,6 +35,7 @@
     { id: 'keyboard',      label: 'Keyboard',      icon: '⌨' },
     { id: 'advanced',      label: 'Advanced',      icon: '🔧' },
     { id: 'marketplace',   label: 'Marketplace',   icon: '🛒' },
+    { id: 'diagnostics',  label: 'Diagnostics',  icon: '📊' },
   ];
 
   let activeCategory = $state<CategoryId>('appearance');
@@ -105,6 +107,8 @@
           <KeyboardSettings />
         {:else if activeCategory === 'marketplace'}
           <MarketplaceTab />
+        {:else if activeCategory === 'diagnostics'}
+          <DiagnosticsTab />
         {/if}
       </div>
     </div>