feat(app): add guided provider setup and env-backed provider import

[townwish4git]

Summary

• Add a guided provider and model selection workflow to /model, including provider browsing, model browsing, setup, and restore.
• Support importing provider credentials from environment-backed local configs to reduce manual key entry during onboarding.
• Keep provider/model state aligned with recent upstream UI event and tool-output updates after rebasing onto the latest main.

Release Highlights

• Guided setup flow for choosing a provider, browsing its available models, and applying the selection in-app.
• Environment-backed provider import suggestions that help users bring existing local credentials into the app with less friction.
• Persistent provider state restoration so previously selected providers and models can be resumed more reliably.
• UI updates across the model setup popup, browser panels, top bar, and hint bar to support the new workflow end to end.

Test Plan

• go test ./internal/app
• go test ./ui/...