refactor(ui): remove debug logs, translate Chinese comments, improve API error logging

[bertheto]

Summary

Code quality cleanup across the frontend JS modules.

Changes

Remove debug console.log in production paths

src/static/js/shared-chat.js

• Remove 4 console.log statements left in sendMessage(): payload dump, response dump, metadata dump, mention count

src/static/index.html

• Remove 7 console.log / console.error statements from �ppendBubble() image rendering path
• Keep console.warn on metadata JSON parse failure (legitimate diagnostic)

Translate Chinese comments to English

src/static/index.html

• openThreadContextMenu(): translate comment about storing right-click position
• getAgentState(): translate 4 inline comments explaining the state machine logic

src/static/js/shared-threads.js

• Remove 2 Chinese inline comments in context menu highlight/clear logic

src/static/js/components/acb-confirm-dialog.js

• Translate all 6 Chinese inline comments in _positionDialog()
• Translate the full JSDoc block on show() from Chinese to English

Improve API error logging

src/static/js/shared-api.js

• Add console.warn on non-2xx HTTP responses (was silently ignored)
• Add console.warn on network errors with method + path context (was swallowed to
  ull)

Tests

• Vitest unit tests: 18/18 passed
• Python unit tests (pytest): 54/54 passed