a11y: harden dialog, controls, and shortcuts

[rwang23]

Audit remediation

Implements the audit's PR-08 slice for #10.

• replaces incomplete tab/listbox/overlay semantics with native buttons and a native persistence dialog;
• provides a tested inert, focus-trapping fallback for older dialog implementations;
• restores invoking focus after cancel/Escape, preserves normal Ctrl/Cmd+C behavior, and gates the global apply shortcut away from dialogs, selections, and editable/interactive controls;
• increases essential explanatory/status text size and contrast, including forced-colors styling;
• adds an isolated Chromium + axe-core serious/critical accessibility regression gate to CI.

Verification

• npm --global=false ci in apps/theme-manager
• npm run test:source
• npm run a11y:check --prefix apps/theme-manager (three consecutive passes; native and fallback dialog keyboard flows, forced colors/reduced motion, axe serious/critical)
• npm run desktop:check (24 Rust tests; no Rust changes after that check)
• node --check for the renderer and accessibility checker
• git diff --check

Independent acceptance

Pass with risks. The new check drives the real renderer through an isolated Chromium session with a mocked Tauri bridge. A packaged Tauri WebView2/WKWebView run and manual visual inspection at 125/150/200% OS scaling remain separate release QA.

Closes #10

[rwang23]

Superseded by #26, now merged into main at e2ed6e8. This branch head is already included in main; closing the draft avoids duplicate review and merge work.