feat(ui): make multi-select discoverable and stop it opening threads

Multi-select already worked by Ctrl+click and Shift+click, but nothing in
the UI said so and every tag action was keyboard-only, so the Ctrl+T tag
dialog could not be reached with a mouse at all.

Adds a select_all action on Ctrl+A, registered like every other action so
it reaches the Edit menu, the shortcut reference and [keys]; a right-click
menu on the thread list built from the same QActions rather than parallel
copies; a selection count in the status bar, which is the part that
actually teaches the feature by acknowledging a selection while it is
being built; and a note in the shortcut dialog for the mouse gestures,
which are view behaviour and cannot appear in the generated table.

A selection gesture must not open mail or mutate it. Selecting several
rows now blanks the message pane and cancels any pending mark-read,
rather than rendering each row swept through and queueing it to be marked
read.

Two Qt behaviours shaped this, both established by probe rather than from
memory:

  - selectAll() emits no currentRowChanged at all and leaves the current
    index invalid.
  - currentRowChanged is emitted BEFORE the selection model is updated.

The second one caused two distinct faults. Collapsing a multi-row
selection back to one row reported the old count, so the guard swallowed
the load and the pane stayed blank; that case is handled in
onSelectionChanged, which sees the true count. And a Ctrl+click taking
the selection from one row to two also reported one, so the thread was
loaded, blanked, and then painted back when the queued reply returned
from the worker. By the third row the id was already cleared and the
reply was discarded, which is why the fault presented as an off-by-one in
the threshold rather than as a race.

Tests cover the synchronous half. The late-reply guard has no test:
MainWindow in tests has no worker, so threadLoaded never fires and the
repaint cannot be reproduced in process. Verified by hand instead.

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

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