feat: add delegated contact picker and UI

[salmonumbrella]

Summary

Adds the delegated-contact frontend workflow: one shared gtd.delegate command, a stable-ID CardDAV contact continuation, immediate person-less fallback, and consistent delegate identity across GTD, inbox, thread, and conversation surfaces.

This is PR 6 of the cumulative command-system roadmap. Please review the last commit only: c772343.

Closes #344.

Changes

• route D, Command-K, row context menus, visible-message controls, single targets, and multiselect through the shared controller
• refresh shared CardDAV state and open the contact picker only for connected external CardDAV
• add debounced stable-ID contact search with stale-response rejection, frozen-target resume, focus trapping/restoration, IME guards, ARIA active descendants, and nearest-row scrolling
• keep the existing dependency-free command architecture and 5.5-row command-palette viewport
• patch successful delegation metadata through account-scoped cached thread/search copies before refresh
• render compact delegate pills on GTD rows, flat and threaded inbox rows, and the open conversation
• localize picker and outcome copy across all seven locales

Testing

• frontend: 1,680 tests passed
• frontend lint: zero warnings
• frontend production build: passed
• Node 22 backend regression: 51 files / 1,014 tests passed
• backend lint: zero warnings
• independent review: no remaining concrete blockers

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