feat(sidebar): distinguish ACP sessions needing input and newly completed turns

[turingcat]

Summary

• show a persistent amber bell and row highlight when an open ACP session needs permission, an answer, structured input, or plan approval
• show a green unread-completion indicator when a background ACP turn finishes, and clear it when the conversation is viewed or a new prompt starts
• keep completion tracking alive while the sidebar drawer is closed and through React Strict Mode effect replays
• add localized labels for all supported languages and expose the new state to assistive technology

Behavior

Sidebar status priority is:

1. needs input
2. unread completion
3. running
4. relative time

Only currently open ACP conversation tabs are tracked. Completion state is intentionally in-memory and is not restored across app restarts. A turn that completes in the active conversation is treated as already read.

Verification

• NODE_OPTIONS=--no-experimental-webstorage ./node_modules/.bin/vitest run (494 files, 7,260 tests)
• ./node_modules/.bin/eslint .
• ./node_modules/.bin/tsc --noEmit
• ./node_modules/.bin/prettier --check ...
• ./node_modules/.bin/next build --webpack

Closes #781