Move hardcoded error/status strings into i18n（错误信息走 i18n）

[Kaka-cheaper]

Goal

Several status / error messages in the viewer are still hardcoded Chinese. Move them all into viewer/src/lib/i18n.ts so they switch with the language toggle.

Why

UI 99% goes through i18n already, but a handful of strings (mostly inside App.tsx's drag handlers and error fallback) escaped. Easy cleanup.

Scope (what is in)

• Grep the viewer/src/ tree for Chinese characters in .tsx/.ts files (excluding example data and i18n.ts itself)
• For each string, add a key in i18n.ts's messages.zh-CN and messages.en, replace the hardcoded string with t('that.key')

Out of scope

• Translation polish (rough English is fine if you're not native)
• AGENTS.md / CHANGELOG / docs (those are author content, not UI)

Files likely involved

• viewer/src/App.tsx
• viewer/src/app/ErrorBoundary.tsx
• viewer/src/app/TopBar.tsx
• viewer/src/lib/i18n.ts

Acceptance criteria

• git grep -P '[\\u4e00-\\u9fff]' viewer/src/{**/*,*}.{ts,tsx} returns nothing (or only what's clearly data)
• Switching language toggle updates all status / error texts
• No regression in npm run build and lint

Estimated effort

~2-3 hours

Hints

useI18n hook is already there and used in many components. Existing keys like topbar.openButton, error.boundaryTitle are good naming examples.

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