Trashing a song mid-extraction stops it, imports stay in the background, and a clear message for read-only portable installs

[thcp]

Fixes for the three reports from @wastedbits after 0.19.1, plus two smaller bugs found along the way.

What works now

Deleting a song while it extracts (#748)

• Moving a song to the Trash while it is queued or extracting now stops it, and the queue goes straight on to the next song. This happens on the server, so the phone's swipe to delete gets it too.
• The song's files stay on disk, the same rule finished tracks in the Trash already follow: only emptying the Trash deletes anything.
• Restore brings it back to the library marked Stopped. It does not go back in the queue on its own. An Extract button on the row runs it again when you choose.
• After a restart a stopped song stays stopped. The "Paused, Start" row from the report no longer appears.
• A cancel now reaches the analyze stage, which used to run to the end regardless.
• Queue rows use the same trash icon as library rows.

Now Playing card during an import (#747)

• An import never takes over the Now Playing card. Progress shows on the library row and in the queue.
• When the first import finishes and nothing is open, it opens by itself, the same as clicking it in the library.

Portable install in Program Files (#746)

• A portable StemDeck in a folder it cannot write to now says so at startup, names the folder, and suggests a writable place such as C:\StemDeck. It no longer fails part way with "Access is denied".
• It does not ask for administrator rights. An elevated window cannot accept drag and drop from Explorer, and every launch would prompt.

Found along the way

• A song cancelled in its last seconds could finish anyway, and every vocal split on it then cancelled itself until a restart (A song cancelled in its last seconds can finish anyway, and then every vocal split on it cancels itself #750).
• On Windows, a cancelled import could leave its folder behind when a file was still locked (On Windows, a cancelled import can leave its folder behind for good #751).

Not in this PR

#749 (stem folders left after deleting songs) is waiting on the reporter. Files are removed when the Trash is emptied, so that one may be expected behaviour rather than a bug.

Testing

• pytest: 2471 passed. ruff, ruff format and bandit are clean.
• Playwright: 441 passed, including new tests for the stopped and Extract flow, a stale Processing row after a restart, and imports staying in the background.
• Rust: cargo fmt, cargo clippy --tests -D warnings and cargo test (131 passed) on Windows, and clippy on Linux through WSL.
• An end-to-end run against a real backend doing real separation, driven with Playwright, passed all 22 checks: trash mid-extraction, the queue moving on, restore, a server restart, Extract to completion, and emptying the Trash.
• Not seen yet: the [Bug]: Errors due to folder permission #746 dialog on a real Program Files install, and a macOS build. CI covers macOS.

Closes #746
Closes #747
Closes #748
Closes #750
Closes #751