refactor: drop unwired ConnectInstanceModal

Commit 7309f44 removed the friend-add trigger because the server now
handles all routing/peering/lookup; the commit message reserved the
modal for 'Connections settings and space-join flows' but neither flow
ever wired it back in. grep across packages/web/src finds only
self-references — pure dead code.

Removing 162 lines of UI plus the stale vi.mock in FriendsPage.test.tsx.
If a per-instance password-re-entry consent UX is ever needed, the
useInstanceConnect hook is the actual abstraction and the modal can be
rebuilt cleanly with current primitives (Modal mobileStyle="fullscreen",
ContextMenuRenderer's bottom-sheet, etc.).

packages/web/src/components/chat/FriendsPage.test.tsx

@@ -108,11 +108,6 @@ vi.mock('../../stores/activityStore', () => ({
   ),
 }));
 
-// Mock ConnectInstanceModal
-vi.mock('../modals/ConnectInstanceModal', () => ({
-  ConnectInstanceModal: () => null,
-}));
-
 const mockNavigate = vi.fn();
 vi.mock('react-router-dom', async () => {
   const actual = await vi.importActual('react-router-dom');