feat(tui): add full terminal onboarding flow with modal provider setup

[penso]

Summary

• add a dedicated crates/tui crate and wire it into the CLI/workspace
• implement blocking onboarding in TUI with modal UX, step navigation, and startup onboarding gate
• add provider configuration flow with inline field editing and model selection support
• make onboarding Esc behavior context-aware: close modal first, quit from top-level onboarding screen
• add/expand TUI onboarding tests to cover rendering, key behavior, and provider/model flows

Validation

Completed

• just format
• cargo +nightly-2025-11-30 clippy -p moltis-tui --all-features --all-targets -- -D warnings
• cargo test -p moltis-tui

Remaining

• Manual end-to-end QA in terminal with real provider credentials

Manual QA

1. MOLTIS_CONFIG_DIR=.moltis/config MOLTIS_DATA_DIR=.moltis/ cargo run --bin moltis -- tui
2. Verify onboarding appears as modal overlay and blocks chat interaction.
3. Verify OpenAI flow:
   • configured provider opens model selection directly
   • unconfigured provider asks for API key, then opens model selection
   • Esc closes provider/modal states, and from top-level onboarding exits app.

[codspeed-hq]

Merging this PR will not alter performance

✅ 39 untouched benchmarks
⏩ 5 skipped benchmarks¹

---

_{Comparing tui-interface (f4cb461) with main (3c24dc3)}

Footnotes

1. 5 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[codecov]

Codecov Report

❌ Patch coverage is 32.77746% with 3507 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/tui/src/app/onboarding.rs	8.77%	1517 Missing ⚠️
crates/tui/src/ui/onboarding.rs	45.89%	560 Missing ⚠️
crates/tui/src/app.rs	7.25%	486 Missing ⚠️
crates/tui/src/onboarding.rs	48.82%	283 Missing ⚠️
crates/tui/src/events/chat.rs	0.00%	223 Missing ⚠️
crates/tui/src/connection.rs	35.02%	115 Missing ⚠️
crates/tui/src/ui/chat.rs	19.04%	102 Missing ⚠️
crates/tui/src/rpc.rs	43.33%	51 Missing ⚠️
crates/tui/src/events/mod.rs	0.00%	44 Missing ⚠️
crates/tui/src/ui/markdown.rs	82.99%	25 Missing ⚠️
... and 7 more

📢 Thoughts on this report? Let us know!