v1.0.26: fix multi-selection reactivity, redesign icon, security review

Fix multi-selection: selectedKeys exposed as StateFlow, collected in
FilesScreen so checkboxes and highlights update correctly on every tap.
fileKey() made public so UI can check membership without ViewModel calls.

Icon: white cloud body with two cyan/teal circular sync arcs (AutoSync
style), deep blue-to-teal gradient background.

Security review clean: no hardcoded credentials, cleartext blocked by
network_security_config, allowBackup=false, path traversal guards in
place on both server responses and local resolution.

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

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

@@ -60,6 +60,7 @@ class FilesViewModel @Inject constructor(
     val isDownloading: StateFlow<Boolean> = _isDownloading
 
     private val _selectedKeys = MutableStateFlow<Set<String>>(emptySet())
+    val selectedKeys: StateFlow<Set<String>> = _selectedKeys
     val isSelectionMode: StateFlow<Boolean> = _selectedKeys.map { it.isNotEmpty() }
         .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), false)
     val selectedCount: StateFlow<Int> = _selectedKeys.map { it.size }
@@ -152,7 +153,7 @@ class FilesViewModel @Inject constructor(
         }
     }
 
-    private fun fileKey(file: SyncFileStateEntity) = "${file.syncPairId}:${file.relativePath}"
+    fun fileKey(file: SyncFileStateEntity) = "${file.syncPairId}:${file.relativePath}"
 
     // ── Download-then-open/share ──────────────────────────────────────────────