feat(gtd): add delegated contact picker

[salmonumbrella]

Summary

Ports the delegated-contact UI from closed PR #343 onto the GTD plugin boundary described in #368. Users can delegate one message or a frozen bulk selection to a CardDAV contact, see the snapshot anywhere the thread appears, or keep the existing personless Delegated workflow.

This PR is stacked on #372 and includes the predecessor commits until they land.

Changes

• Add a plugin-owned delegation controller that freezes and deduplicates up to 100 target IDs before checking CardDAV or opening UI.
• Open an accessible searchable contact dialog when CardDAV is connected. When it is not connected, delegate without a person immediately.
• Handle stale searches, keyboard wrapping, Escape, Enter, focus trapping/restoration, retry, empty results, and an explicit “delegate without person” action.
• Add neutral core seams for selected context-menu IDs, reading-pane actions/metadata, and GTD-entry metadata. Core does not import GTD.
• Add “Delegate…” to the context menu, reading pane, and detached message windows; show contact/age pills wherever the thread appears.
• Keep uppercase D as the contact shortcut and lowercase d as personless Delegated classification, scoped to the active thread target.
• Patch successful snapshots immediately, then reconcile from the plugin metadata endpoint.
• Localize the picker, outcomes, shortcut, and accessibility copy in all seven shipped locales.

Blocked by #372.

Refs #343 and #368

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