feat(device-sync): manifest, cancel, font picker + sync status fix

- Write psysonic-sync.json to device after sync/deletions; auto-import
  on folder select when localStorage is empty (cross-platform handoff)
- Add cancel button during active sync: AtomicBool flag per job,
  tasks bail after acquiring semaphore, cancelled state in UI
- Fix sync status staying "pending": normalize template path separators
  to OS separator (Windows: '/' → '\') so compute_sync_paths and
  list_device_dir_files produce matching strings for Set comparison
- Add Geist and JetBrains Mono as variable fonts; font picker collapsible
- Fix device mount detection on Windows: removable drive letters (E:\)
  were incorrectly skipped alongside system roots; strip \?\ prefix
  from canonicalized paths before mount-point comparison

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/locales/en.ts

@@ -1088,6 +1088,7 @@ export const enTranslation = {
     deleteFromDevice: 'Mark for deletion ({{count}})',
     confirmDelete: 'Delete {{count}} item(s) from the device: {{names}}?',
     deleteComplete: '{{count}} item(s) removed from device.',
+    manifestImported: 'Imported {{count}} source(s) from device manifest.',
     selectedSources: 'Selected Sources',
     noSourcesSelected: 'No sources selected. Click items in the browser to add them.',
     clearAll: 'Clear all',
@@ -1107,6 +1108,8 @@ export const enTranslation = {
     fetchError: 'Failed to fetch tracks from server.',
     syncComplete: 'Sync complete.',
     dismiss: 'Dismiss',
+    cancelSync: 'Cancel',
+    syncCancelled: 'Sync cancelled ({{done}} / {{total}} transferred).',
     statusSynced: 'Synced',
     statusPending: 'Pending',
     statusDeletion: 'Deletion',