feat(ui): Add thinking/reasoning block visibility toggle

[sauerdaniel]

Summary

Add a keyboard shortcut Ctrl+i to toggle thinking/reasoning block visibility in the TUI.

Fixes #13789

Problem

No quick way to toggle thinking block visibility while using the TUI — users must navigate through menus.

Solution

Add Ctrl+i keybinding that toggles thinking mode visibility in the session turn component, consistent with other TUI quick-settings shortcuts.

Changes

• packages/ui/src/components/session-turn.tsx — Add Ctrl+i toggle for thinking block visibility with proper mouse event typing

Testing

• TypeScript compilation passes (bun turbo typecheck)
• Unit tests pass

AI-Assisted

Yes

Design review note

This touches TUI UI behavior; per CONTRIBUTING.md, maintainer design review is requested before merge.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate Found

PR #7868: feat(app): add /thinking command to toggle reasoning visibility

• feat(app): add /thinking command to toggle reasoning visibility #7868

Why it's related: Both PRs address the same core feature - toggling thinking/reasoning block visibility. However, PR #7868 uses a /thinking command approach while PR #9139 (current) adds a UI toggle button. These may be complementary approaches or duplicative depending on the intended implementation strategy.

Also related (but older/different scope):

• PR Feature/OpenAI compatible reasoning #5531: Feature/OpenAI compatible reasoning - Feature/OpenAI compatible reasoning #5531 (earlier reasoning feature work)

[sauerdaniel]

Rebased onto latest dev and resolved merge conflicts.\n\nNo functional change beyond the thinking-visibility toggle; also typed mouse event handlers to satisfy TS.\n\nPlease re-run checks / re-review when convenient.

[sauerdaniel]

Re: potential overlap with #7868 — different approach. #7868 adds a /thinking slash command (requires session.tsx changes), while this PR adds a UI toggle directly in the thinking block component (session-turn.tsx only). They're complementary features (command vs inline toggle) that happen to touch the same file.