feat(web): add persistent theme/language preferences and accessibility/mobile hardening

[sowen1023]

Parent

• [Roadmap] OpenPI Web：像 Codex / Kimi Code 一样的本地浏览器 Agent 工作台 #76

Problem

The current Web UI follows browser language detection and one fixed visual presentation. It has no user-facing theme or language controls, no reviewed persistence contract for those choices, and only initial responsive/reduced-motion coverage. Keyboard navigation, focus management, screen-reader semantics, contrast, zoom, and narrow mobile layouts have not received a complete real-browser audit.

These are recoverable polish gaps, but together they prevent the workbench from being a dependable daily interface for a broader range of users.

Value

• Let users choose a readable appearance and language consistently.
• Make all core workbench operations usable without a pointer.
• Catch mobile and accessibility regressions with repeatable evidence.

Approach constraints

• Package-owned theme/UI preferences follow the OpenPI setup configuration contract: canonical config, setup status, SETUP.md, README defaults, and tests stay synchronized.
• Define which transient browser state, if any, may remain browser-local; do not create an accidental second package configuration store.
• Preserve system preference fallback and reduced-motion behavior.
• Theme and language changes must not reload, duplicate, or mutate the active Session.
• Avoid color-only status communication and preserve semantic labels.
• Validate real browser behavior in addition to static source assertions.

Acceptance criteria

• Users can choose documented supported themes plus system/default behavior.
• Users can explicitly choose supported languages plus automatic detection.
• Preferences survive reload according to the documented canonical storage contract.
• No credential, Session transcript, or workspace path is persisted with UI preferences.
• Sidebar, dialogs, model picker, search, composer, transcript, tool details, and activity panels are fully keyboard operable.
• Focus is trapped/restored correctly for dialogs and remains visible.
• Accessible names, roles, live updates, status text, contrast, zoom, and reduced motion are audited.
• Core flows remain usable at documented narrow viewport widths and with touch input.
• Automated accessibility checks and real-browser desktop/mobile interaction tests are added.
• Setup/config documentation and drift guards are updated for package-owned preferences.
• bun run check and bun run test pass.

Out of scope

• Native mobile applications.
• Pixel parity with another product.
• Remote synchronization of personal UI preferences.