feat: add safe GTD classification undo

[salmonumbrella]

Summary

Ports the GTD classification undo from closed PR #333 onto the plugin boundary described in #368. Classification still copies the message into the configured GTD folder immediately; when the IMAP server returns an exact UIDPLUS destination, that new copy can now be undone without touching Inbox, another label, or another message in the thread.

Changes

• Return whether GTD classification created a copy and, when available, its exact destination UID.
• Add a plugin-owned undo endpoint that rechecks the current state-folder mapping and matches account, folder, UID, and RFC Message-ID before deleting anything. Replays and stale tokens are safe no-ops.
• Do not offer undo when the server lacks UIDPLUS, the label already existed, or the source has no stable Message-ID.
• Add undo notifications to the GTD runtime and context menu, plus Ctrl+Z / Cmd+Z for the newest GTD classification.
• Translate the shortcut and failure state in all seven locales.
• Keep explicit unclassification unchanged. This does not port delayed archive or any other core Inbox triage behavior from feat: undo inbox triage actions #333, and it adds no migration.

Testing

• Backend: npm test — 1,117 tests passed across 61 files.
• Backend: npm run lint and npm run lint:plugins.
• Frontend: full node --test suite, npm run lint, and npm run build.
• Regression coverage includes UIDPLUS and non-UIDPLUS copies, existing labels, missing Message-ID, exact undo, replay, stale folder mappings, forged UIDs, one-shot frontend callbacks, and the GTD undo shortcut.
• Roborev reviewed the final tree with no issues found.

Refs #368

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