feat: add MainWindow wiring query, list, message, and sync

Wires the worker thread, thread list, message pane, sync process and undo
stack together, and replaces the placeholder main() with real startup:
custom URL schemes registered before QApplication, a libnotmuch ABI check,
and config loading.

Four fixes against the drafted version:

- onWorkerError() only set a status label. Its own comment elsewhere
  claimed it reverted the optimistic update, and the spec requires that;
  it did not, so a rejected write left the list showing a tag the database
  never received. The pending change is now recorded and rolled back, and
  a confirmed tagsApplied clears it so a later unrelated error cannot undo
  a write that succeeded.
- runCurrentQuery() cleared the model but left the undo stack pointing at
  rows that no longer exist. Undoing after a new query would have written
  to the database while the visible list stayed put. The stack is cleared
  with the model.
- m_currentMessages was assigned on every thread load and never read.
  Removed.
- buildUi() connected sync output to m_syncLog and errors to m_statusLabel
  before either existed. Both are constructed before the wiring now.

cidPrefix generation lives here, this being its only producer in the
application, and is pinned by tests: it must never contain '!' and must be
distinct per message, which are the invariants the cid: namespacing rests
on. A second test holds registeredActionNames() against
KeyMap::knownActions(), since those two hand-maintained lists drifting
either way silently breaks a user's key binding. Mutation-verified that
dropping an action fails the test by name.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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