When the editor mode changes (e.g. Esc → Normal, i → Insert),
the mode tag in the status bar flashes to the accent colour
for ~300ms. Provides instant visual feedback for mode changes.
- Editor::mode_flash: Option<Instant> field
- Editor::trigger_mode_flash() / mode_flash_progress()
- Editor::set_mode() helper that triggers the flash on actual
mode changes (no-op when setting the same mode)
- All self.mode = Mode::* sites in handlers.rs updated to use
set_mode() so the flash triggers consistently
- status_spans uses accent + cursor_fg for the mode tag while
the flash is active
Added 1 unit test verifying flash triggers on mode change and
NOT on setting the same mode.
Tests: 1397 pass (was 1396), zero warnings.
vasilito
c9ad1e8deb