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/Cargo.toml

@@ -57,6 +57,10 @@ tauri-plugin-notification = "2"
 mozjpeg = "0.10.13"
 tauri-plugin-updater = "2"
 tauri-plugin-process = "2"
+tauri-plugin-log = "2"
+tauri-plugin-single-instance = "2"
+tauri-plugin-window-state = "2"
+log = "0.4"
 
 # ── Dev-mode performance ────────────────────────────────────────────────────
 # opt-level=1 on the main crate keeps incremental compile short.