fix(updater): retry the check and download, report progress

A friend's in-app update failed ten times ("error sending request for
url (…nsis.zip)") against a busy Gitea before the 11th worked.

- check_for_update / install_update retry transport errors (Reqwest,
  Network, ReleaseNotFound) with 3 s / 8 s / 15 s backoff — four tries
  per click. Signature or install errors fail at once, never masked.
- Every request gets a 600 s timeout; reqwest has none by default, so a
  stalled connection could leave the UI on "installing" forever.
- Download and install are separate steps so a retry never re-installs.
- `lotus-update-progress` events (downloading %, retrying + wait,
  installing) drive the new progress text in Settings.
- Errors are prefixed `check:` / `download:` / `install:` so the UI says
  which step failed (it used to call a failed download "Update check
  failed").
- Adds tokio (time feature only; already in the tree via tauri).
- Bump cinny to 568f218f (the matching UI).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src-tauri/Cargo.toml

@@ -36,6 +36,8 @@ custom-protocol = [ "tauri/custom-protocol" ]
 tauri-plugin-updater = "2"
 tauri-plugin-single-instance = "2"
 tauri-plugin-autostart = "2" # P6-1 launch-on-login
+# Update retry backoff (already in the tree via tauri; adds only the timer).
+tokio = { version = "1", features = ["time"] }
 
 [target.'cfg(target_os = "linux")'.dependencies]
 # P6-1 desktop parity: screensaver inhibit (no-sleep in calls) + Unity launcher