Complete frontend migration to unified Pi harness (Phase 7 + Phase 9)

- Delete legacy hooks: useClaude.ts, useOpenCode.ts, usePiMono.ts, App.backup.tsx
- Update all components to use usePi instead of legacy hooks
- Replace useVisiblePiMonoModels/useClaudeModels/useOpenCodeModels with useVisiblePiModels
- Migrate from LegacyChatMessage to ChatMessage type throughout
- Update SessionBar to remove provider and archive props
- Simplify ChatDetailPanel to Pi-only (remove Claude/OpenCode components)
- Fix useChatSessions calls (remove provider parameter)
- Update user-settings types: provider now only 'pi' instead of legacy values
- Update PI_HARNESS_REBUILD.md to mark phases complete

src/apps/officer-web/Screens/Dashboard/ChatHistory/Widget.tsx

@@ -15,7 +15,7 @@ export const ChatHistory = () => {
           <ul className="space-y-0.5">
             {sessions.map((session) => (
               <li
-                key={`${session.provider}-${session.id}`}
+                key={session.id}
                 className="flex items-center gap-2 px-2 py-1.5 rounded-md hover:bg-duck-dark/5 group"
               >
                 <Link
@@ -46,7 +46,7 @@ export const ChatHistory = () => {
                   </div>
                 </Link>
                 <button
-                  onClick={() => deleteSession(session.provider, session.id)}
+                  onClick={() => deleteSession(session.id)}
                   className="shrink-0 p-1 rounded text-duck-dark/20 md:opacity-0 md:group-hover:opacity-100 hover:text-red-500 transition-opacity cursor-pointer"
                 >
                   <Trash2 className="h-3.5 w-3.5" />