Add configuration backup/restore with settings modal

Backend: GET /api/v1/system/backup bundles all 11 store JSON files into a
single downloadable backup with metadata envelope. POST /api/v1/system/restore
validates and writes stores atomically, then schedules a delayed server restart
via detached restart.ps1 subprocess.

Frontend: Settings modal (gear button in header) with Download Backup and
Restore from Backup buttons. Restore shows confirm dialog, uploads via
multipart FormData, then displays fullscreen restart overlay that polls
/health until the server comes back and reloads the page.

Locales: en, ru, zh translations for all settings.* keys.

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

server/src/wled_controller/api/schemas/system.py

@@ -68,3 +68,14 @@ class PerformanceResponse(BaseModel):
     ram_percent: float = Field(description="RAM usage percent")
     gpu: GpuInfo | None = Field(default=None, description="GPU info (null if unavailable)")
     timestamp: datetime = Field(description="Measurement timestamp")
+
+
+class RestoreResponse(BaseModel):
+    """Response after restoring configuration backup."""
+
+    status: str = Field(description="Status of restore operation")
+    stores_written: int = Field(description="Number of stores successfully written")
+    stores_total: int = Field(description="Total number of known stores")
+    missing_stores: List[str] = Field(default_factory=list, description="Store keys not found in backup")
+    restart_scheduled: bool = Field(description="Whether server restart was scheduled")
+    message: str = Field(description="Human-readable status message")