Accessibility pass: ARIA on menus/autocomplete/tabs, aria-live on errors, visible focus rings

Part of #68 (Roadmap to 1.0.0) — should-have. The substantial one.

Problem

The app has almost no accessibility affordances — ~11 a11y attributes across all of src/ (mostly tabindex, one aria-hidden). For a complex SQL IDE this is a significant screen-reader and keyboard-navigation gap.

Scope

• ARIA semantics on the custom widgets so assistive tech announces them correctly:
  • autocomplete dropdown → listbox/option + aria-activedescendant (src/ui/editor-complete.js)
  • File menu and the user/save popovers → menu/menuitem (src/ui/file-menu.js, app.js anchoredPopover)
  • query tab strip → tablist/tab (src/ui/tabs.js)
• aria-live on the toast / error region so query errors and "session expired" are announced, not just shown (src/ui/toast.js).
• Visible keyboard-focus rings everywhere outline:none is set without a replacement (src/styles.css:158,165,320,408,454,528,535,…). Today, tabbing through some controls shows no focus indicator.

Acceptance

• Screen reader announces the autocomplete, menus, and tabs with correct roles/state.
• Errors/toasts are announced via aria-live.
• Every interactive control shows a visible focus ring on keyboard focus.
• npm test green at the per-file coverage gate.

Largest of the 1.0 should-haves; worth its own focused pass + a screen-reader/keyboard test.

Folded in from the roadmap (#68) nice-to-haves:

• Honor prefers-reduced-motion — a @media (prefers-reduced-motion: reduce) block that disables/limits transitions and animations (chart updates, hovers) for users who've set reduce-motion in their OS.

まず src/ui/editor-complete.js、src/ui/file-menu.js、app.js、src/ui/tabs.js、src/ui/toast.js、および src/styles.css にある、記載されている outline:none ルールを読んでください。npm test を実行してファイルごとのカバレッジゲートを理解し、その後、記載されているスクリーンリーダー、キーボードフォーカス、aria-live、モーション削減の受け入れ基準を、言及されているスクリーンリーダー／キーボードテストで確認してください。