Select-all in history, and a confirmation before batch delete

[TN019]

Requested: a select-all for the history page. With hundreds of cards, ticking them one at a time to batch-translate or batch-delete isn't practical.

Select all

A tristate checkbox in the toolbar:

state	shows
nothing selected	☐ Select all
some selected	▣ 1 of 3 selected
all selected	☑ 3 of 3 selected

Clicking it from the partial state selects the remainder rather than clearing — continuing upward is the likelier intent when you have already ticked a few. Only the fully-checked state clears.

Details worth knowing:

• It listens on clicked, not toggled. clicked fires only for a real click, so the programmatic setCheckState() that keeps the box in sync with the cards can't loop back into the handler.
• A bulk toggle suspends the toolbar rebuild and does it once at the end, instead of once per card — this page can hold several hundred rows.
• Fixed along the way: refresh() called _sync_delete_button() at the top, before the cards were built, so it always measured an empty page. It now syncs again after the loop.

Batch delete asks first

Not requested, and easy to drop if it isn't wanted — but select-all turns "erase the entire history index" into two clicks, and delete has no undo. The batch path now confirms; the per-card ✕ is unchanged and still deletes on one click.

It removes records only, never the subtitle files on disk, and the dialog says so. The buttons carry our own i18n labels because Qt's standard buttons follow the system locale rather than the app's language setting.

This change is load-bearing enough to have hung an existing test — test_history_delete_single_and_batch drives the batch path and blocked on the real modal — which is updated to confirm explicitly.

Tests

204 passed / 5 skipped. Three new: the tristate round trip including the partial-state behaviour, the box hiding when there is nothing to select, and both branches of the confirmation plus a single-row delete not triggering it.