v1.0.58: Files tab → dual-mode file explorer (Phone + Cloud)

Replace the synced-files list with a proper file explorer:
- Phone tab: browse all of internal storage with quick-access shortcuts
  (Camera, Downloads, Documents, Pictures, Music, Videos), breadcrumb
  navigation, search, tap folder to enter, tap file to open/share
- Cloud tab: browse connected cloud accounts, account switcher chips for
  multiple accounts, breadcrumb navigation, search, tap file to
  download+open

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

app/src/main/kotlin/com/syncflow/ui/files/FilesViewModel.kt

@@ -40,6 +40,9 @@ class FilesViewModel @Inject constructor(
     val pairs: StateFlow<List<SyncPairEntity>> = syncPairDao.observeAll()
         .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), emptyList())
 
+    val accounts = accountRepository.observeAll()
+        .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), emptyList())
+
     private val _selectedPairId = MutableStateFlow<Long?>(null)
 
     val selectedPair: StateFlow<SyncPairEntity?> = combine(_selectedPairId, pairs) { id, list ->
@@ -158,6 +161,31 @@ class FilesViewModel @Inject constructor(
 
     fun fileKey(file: SyncFileStateEntity) = "${file.syncPairId}:${file.relativePath}"
 
+    fun openCloudFile(accountId: Long, remotePath: String) {
+        viewModelScope.launch {
+            val account = accountRepository.getAccount(accountId) ?: run {
+                _fileAction.emit(FileAction.Error("Account not found"))
+                return@launch
+            }
+            val provider = providerFactory.create(account)
+            val fileName = remotePath.substringAfterLast('/')
+            val cacheFile = File(context.cacheDir, "syncflow_open/$fileName")
+            cacheFile.parentFile?.mkdirs()
+            _isDownloading.value = true
+            try {
+                cacheFile.outputStream().use { out ->
+                    provider.downloadFile(remotePath, out) { }.getOrThrow()
+                }
+                _fileAction.emit(FileAction.Open(cacheFile))
+            } catch (e: Exception) {
+                Timber.e(e, "Cloud open failed: $remotePath")
+                _fileAction.emit(FileAction.Error("Cannot open: ${e.message}"))
+            } finally {
+                _isDownloading.value = false
+            }
+        }
+    }
+
     // ── Download-then-open/share ──────────────────────────────────────────────
 
     private fun downloadAndOpen(file: SyncFileStateEntity) {