fix(tui): switch theme appearance with arrow keys

[hetaoBackend]

Change

The /theme picker now uses Left and Right to move between Light, Auto, and Dark appearance choices. The selection stops at either end, and a no longer changes appearance. The footer shows the arrow-key control.

Validation

• pnpm exec vitest run --config vitest.oss.config.mjs packages/tui/test/unit/tui/features/settings/theme-picker.test.ts — 12 passed.
• pnpm verify on d3ffa734e15254634075c01f54d23dccdc8bbbf2 — 14 gates passed on macOS.
• git diff origin/main...HEAD --check — passed.
• Performance: basic PR suite; perf:full is not required for this picker-only input change.
• NOT RUN: Windows/Linux, a manual terminal interaction, and live-service testing.

Publication and contribution checks

• I have permission to contribute these changes under the existing licenses applicable to the changed files/packages; no imported material is included.
• No credentials, account data, real user content, internal source history or private review material is included.
• No source files were added or removed; no test-suite registration or inventory update is needed.
• The picker footer documents the new keys; shared documentation and capability records do not require changes. Offline results are not described as live-service acceptance.

Maintainer handoff

Publication scope or license changes: none.

Shared-source port: pending; this PR changes only the public TUI picker.

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