fix(a11y): add aria-label to icon-only buttons across plugins

[claude]

Summary

• Add aria-label to 15+ icon-only buttons across React components and imperative DOM plugins
• Replace hardcoded English strings in plugin buttons with i18n.t() calls (i18n compliance)
• Add 15 new plugin.* translation keys to editor.json for all 10 locales

Files changed

React components (aria-label added):

• HistoryView.tsx — revert/delete snapshot buttons
• StatusBarRight.tsx — source/rich-text mode toggle

Plugin buttons (aria-label + i18n):

• diagramExport.ts — Export PNG
• mermaidPreviewDOM.ts — Zoom out/in
• previewHelpers.ts — Copy/Cancel/Save
• renderMarkmapPreview.ts — Fit to view
• sourcePeekHeader.ts — Live preview/Cancel/Save
• ImagePasteToastView.ts — Insert/Dismiss
• mermaidPanZoom.ts — Reset zoom
• aiSuggestion/tiptap.ts — Accept/Reject (already had aria-label, now i18n'd)

Locale files (15 new keys each):

• src/locales/{en,de,es,fr,it,ja,ko,pt-BR,zh-CN,zh-TW}/editor.json

Test plan

• pnpm check:all passes (lint, i18n, tests, build)
• Tab through editor buttons with screen reader — all announce their purpose
• Verify plugin buttons show translated labels in non-English locales

Closes #443

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