feat(hardening): production logging, single-instance, CSP, and scoped asset protocol

Phase 4 of the 0.1.0 release prep:
- tauri-plugin-log: rotating file log (5 MB, Info) in the app log dir plus
  stdout; all 54 backend println!/eprintln! sites migrated to log macros so
  release builds finally produce diagnostics
- tauri-plugin-single-instance (registered first): second launches focus the
  existing window instead of racing the same SQLite DB with a second worker
  fleet; tauri-plugin-window-state persists window geometry
- real CSP replacing null (scripts self-only, media/images via the asset
  protocol, IPC endpoints whitelisted, object-src none) with a devCsp
  variant for Vite HMR
- asset protocol scope narrowed from '**' to thumbnails statically plus
  per-folder runtime grants (setup, add_folder, update_folder_path)

Panic audit (plan item) concluded with no changes: worker loops already
catch and log all Result errors; remaining unwraps are startup fail-fast,
poisoned-lock convention, infallible-by-construction, or test code.

src-tauri/src/media.rs

@@ -43,22 +43,23 @@ impl MediaTools {
         }
         auto_download_with_progress(|event| match event {
             FfmpegDownloadProgressEvent::Starting => {
-                println!("Downloading bundled FFmpeg...");
+                log::info!("Downloading bundled FFmpeg...");
             }
             FfmpegDownloadProgressEvent::Downloading {
                 total_bytes,
                 downloaded_bytes,
             } => {
-                println!(
+                log::info!(
                     "Downloading bundled FFmpeg: {}/{} bytes",
-                    downloaded_bytes, total_bytes
+                    downloaded_bytes,
+                    total_bytes
                 );
             }
             FfmpegDownloadProgressEvent::UnpackingArchive => {
-                println!("Unpacking bundled FFmpeg...");
+                log::info!("Unpacking bundled FFmpeg...");
             }
             FfmpegDownloadProgressEvent::Done => {
-                println!("Bundled FFmpeg ready.");
+                log::info!("Bundled FFmpeg ready.");
             }
         })
     }