fix: harden builtin TUI theme readability

[xy200303]

Summary

• Address issue 改善 macOS TUI 内置主题的文字可读性 #21 by hardening the five builtin TUI themes so low-emphasis text and structural rails stay readable after /theme switches on macOS terminals.
• Keep transcript badges and the summary label readable by rendering their text with the shared foreground color instead of theme-specific accent colors that can be too dark on some badge backgrounds.
• Add a regression test that enforces minimum contrast thresholds for builtin theme surfaces, selection states, dividers, and badge backgrounds.
• Keep the branch current with main and make the newly merged Windows-sensitive tests pass under normal Windows permissions.

What Changed

• Tuned builtin muted and line colors in src/tui/theme.ts across default, sage, pop, helix, and steel.
• Updated transcript badge rendering and the SUMMARY label in src/tui/app.tsx to use theme.foreground for badge text while preserving existing badge background and rail colors.
• Expanded the /theme status copy so users can recover quickly with /theme default when a custom or builtin theme is hard to read.
• Added src/tui/theme.test.ts to verify contrast constraints for foreground, muted text, selections, dividers/rails, and badge backgrounds.
• Updated Windows-sensitive utility tests so pnpm check passes under normal Windows permissions.

Verification

• pnpm exec vitest run src/tui/theme.test.ts
• pnpm exec vitest run packages/utils/src/path.test.ts packages/utils/src/shell.test.ts
• pnpm exec tsc --noEmit --pretty false
• pnpm exec prettier --check src/tui/theme.ts src/tui/app.tsx src/tui/theme.test.ts packages/utils/src/path.test.ts packages/utils/src/shell.test.ts docs/skills/pr-review-decision.md
• pnpm check

Notes

• Closes 改善 macOS TUI 内置主题的文字可读性 #21.

[ZouR-Ma]

Thanks for the contribution!

To get this ready to merge:

• Reference an issue in the description with Closes #<number> (our link-check CI requires it; open one first if needed).
• Make sure pnpm check passes locally (lint, type-check, dependency/dead-code guards, formatting, and tests — also enforced in CI).

Full conventions:
https://github.com/stepfun-ai/Step-Realtime-CLI/blob/main/CONTRIBUTING.md

[xy200303]

Thanks! I've addressed the feedback on this PR:

• Linked the PR description with Closes #21.
• Updated the PR description with the current verification steps.
• Confirmed pnpm check passes locally and all PR checks are green.

This should be ready for review/merge when you have a chance.