feat(providers): redesign setup and connect model pickers

[georgi]

What changed

Model pickers now open provider setup when no configured provider supports the requested model type, and every picker includes an Add providers action. Provider discovery finishes before prompting, and local, OAuth, and managed providers count as configured. The setup dialog groups account sign-in and API keys, filters choices by task, and refreshes provider/model caches after connecting. The Providers settings page uses grouped rows with clearer status and actions. OpenAI sign-in is offered for chat, while image, speech, and embedding setup requires an API key.

Verification

• npm run test:affected — 539 suites passed, 4,610 tests passed, 3 skipped. Run with localhost access for UI image fixtures.
• NODE_OPTIONS=--max-old-space-size=8192 npm run typecheck
• npm run lint
• npm run dev:nodetool -- harness gate --base origin/main — 3 selfchecks passed.
• Inspected the provider dialog at desktop and phone widths and the model picker in the running app.

The automatic-opening assertion was inverted once in useModelProviderSetup.test.tsx and failed as expected, then restored and rerun successfully:

Expected number of calls: 0
Received number of calls: 1
Test Suites: 1 failed, 1 total
Tests: 8 failed, 9 passed, 17 total

Screenshots

Desktop provider setup

Mobile provider setup