refactor(device-sync): G.75 — extract drives hook + source-statuses hook + migration + execution orchestrators (cluster) (#642)

Four-cut cluster pulling the orchestrators out of DeviceSync.tsx.
1179 → 905 LOC (−274).

useDeviceSyncDrives — drives state + drivesLoading + refreshDrives
callback + the 5 s polling useEffect + the activeDrive memo that
matches targetDir against any detected drive's mount_point.
Returns { drives, drivesLoading, activeDrive, driveDetected,
refreshDrives }.

useDeviceSyncSourceStatuses — owns sourcePathsMap state + the
useEffect that computes per-source paths through compute_sync_paths
(parallel for all sources, with the cancellation guard), and the
derived sourceStatuses Map keyed on 'synced' / 'pending' /
'deletion'.

runDeviceSyncMigration — runDeviceSyncMigrationPreview (read v1
manifest's filenameTemplate, fetch album-source tracks, compute
new paths via Rust + old paths via JS legacy template, diff into
pairs + collisions + unchanged) and runDeviceSyncMigrationExecute
(invoke rename_device_files, bump manifest to v2, rescan device).
Exports MigrationPhase / MigrationPair / MigrationResult types so
the page state stays typed.

runDeviceSyncExecution — runDeviceSyncSummaryPrompt (input
validation + invoke calculate_sync_payload through the subsonic
client) and runDeviceSyncExecute (delete pending sources, re-write
playlist m3u8 even when nothing to download, fire sync_batch_to_device
with the right toast variants on space / mount / generic errors).
SyncDelta type moves with it.

DeviceSync drops the inline definitions; closeMigration stays
inline (six lines, no real win extracting it). Pure code move
otherwise — no behaviour change.

src/hooks/useDeviceSyncDrives.ts

@@ -0,0 +1,45 @@
+import { useCallback, useEffect, useMemo, useState } from 'react';
+import { invoke } from '@tauri-apps/api/core';
+import type { RemovableDrive } from '../utils/deviceSyncHelpers';
+
+export interface DeviceSyncDrivesResult {
+  drives: RemovableDrive[];
+  drivesLoading: boolean;
+  activeDrive: RemovableDrive | null;
+  driveDetected: boolean;
+  refreshDrives: () => Promise<void>;
+}
+
+export function useDeviceSyncDrives(targetDir: string | null): DeviceSyncDrivesResult {
+  const [drives, setDrives] = useState<RemovableDrive[]>([]);
+  const [drivesLoading, setDrivesLoading] = useState(false);
+
+  const refreshDrives = useCallback(async () => {
+    setDrivesLoading(true);
+    try {
+      const result = await invoke<RemovableDrive[]>('get_removable_drives');
+      setDrives(result);
+    } catch {
+      setDrives([]);
+    } finally {
+      setDrivesLoading(false);
+    }
+  }, []);
+
+  // Fetch drives on mount, then poll every 5 seconds
+  useEffect(() => {
+    refreshDrives();
+    const interval = setInterval(refreshDrives, 5000);
+    return () => clearInterval(interval);
+  }, [refreshDrives]);
+
+  // Detect if the current targetDir is on a detected removable drive
+  const activeDrive = useMemo(() => {
+    if (!targetDir) return null;
+    return drives.find(d => targetDir.startsWith(d.mount_point)) ?? null;
+  }, [targetDir, drives]);
+
+  const driveDetected = activeDrive !== null;
+
+  return { drives, drivesLoading, activeDrive, driveDetected, refreshDrives };
+}