Spawn file operations on background threads and show the existing
ProgressDialog (gauge, sparkline, ETA, cancel button) while the
operation runs. The event loop polls the result channel every frame
and applies the outcome when the thread finishes.
- DialogState::Progress variant added
- PendingProgressOp tracks kind/sources/destination/result channel
- spawn_op_with_progress() spawns thread + shows modal progress
- tick_progress() polls channel, handles Ok/Err/Empty/Disconnected
- ProgressDialog Cancel key (Enter) cancels the OpHandle
- Tab toggles focus on the cancel button
- Wired into app.rs event loop (re-render every tick while running)
Tests: 1381 pass, zero warnings.
vasilito
9024b87934