Name the structure toggle for the import it changes

[thcp]

Closes #634

A reporter clicked "Song structure" on an open track, watched the button change colour, and saw the sections bar stay exactly where it was.

What is actually going on

The button and the bar are unrelated. The button is a server-side flag the pipeline reads once per job, so it can only ever affect the next import. The bar is a permanent part of the studio, showing whatever sections a track already has, whether detected at import or added by hand. Nothing the button does can reach a track that has already been separated.

Two things made that read as a fault:

• The name described the structure rather than the work, so it looked like the switch that shows and hides the bar.
• The toggle clears itself every time a song loads, because an inference pass costs minutes of CPU and should not be inherited from the last track. So the user finds it off after an import, clicks it expecting off, and gets on.

What changed

Wording only, no behaviour.

• The button is "Detect structure", and the tooltip says it applies to the next import rather than to an open track.
• The tooltip also points at the control that does own the bar: "Sections" in the Collapse row, which has shipped since before this was reported. The reporter was looking for it and never found it.
• structure.toggleAria is new. The title is three sentences, which is right for a tooltip and wrong for a screen reader, so the aria label is short instead of reusing the title key.
• Ten language tables, each naming its own locale's menu path rather than the English one.

Tests

Two, both in the existing spec. The first is the reported behaviour: the toggle leaves the sections bar alone in both directions, while the panel toggle still hides and shows it. The second pins the label, since naming the import is the whole fix and is otherwise the sort of thing a later tidy-up would undo.

Verified

pytest (1078 passing), ruff check and format, bandit, the full Playwright suite (183 passing), node --check, the JS unit tests, i18n coverage clean at 525 keys per table, uv.lock untouched, and cargo fmt, clippy and test unchanged at 99.