refactor(types): eliminate all as any type assertions (30 → 0)

- Replace MouseEvent-only handlers with MouseEvent | KeyboardEvent unions in multi-select and ChatTabBar
- Define LegacyInstalledPlugin type for backward compatibility in plugin-api
- Create getTokenValue() helper for type-safe theme token access in AppearanceTab
- Define ModelConfig interface for model configuration type safety in modelProvider
- All modifications maintain exact business logic equivalence
- Build passes with zero TypeScript errors (bun run build ✅)
- LSP diagnostics clean on all modified files

dashboard/src/routes/chat/ChatTabBar.tsx

@@ -7,7 +7,7 @@ interface ChatTabBarProps {
   tabs: ChatTab[]
   activeTabId: string
   onSwitch: (tabId: string) => void
-  onClose: (tabId: string, e?: React.MouseEvent) => void
+  onClose: (tabId: string, e?: React.MouseEvent | React.KeyboardEvent) => void
   onAddVirtual: () => void
 }
 
@@ -55,7 +55,7 @@ export function ChatTabBar({
                   onKeyDown={(e) => {
                     if (e.key === 'Enter' || e.key === ' ') {
                       e.preventDefault()
-                      onClose(tab.id, e as any)
+                      onClose(tab.id, e)
                     }
                   }}
                 >