feat(updater): fixed-URL auto-update channel + silent retries

The auto-update system used a per-version publish URL
(releases/download/v${version}), so each installed build only ever
checked its own release page for new versions. To deliver an update we
had to manually copy the new manifest into every old release — easy to
forget, and any half-uploaded state showed users red "check failed"
banners.

Architectural fix:

- New rolling 'update-channel' Gitea release. publish.url is now a
  fixed path (.../releases/download/update-channel) that never moves.
- release.ps1 uploads each new build to three places:
    1. vX.Y.Z          (historical archive + changelog)
    2. update-channel  (what every client polls)
    3. -BridgeTags     (transition: also fill in old releases so users
                       still on those versions can find the new build)
- upload-release-assets.ps1 gains -AssetVersion to upload version-X.Y.Z
  artifacts into a non-version tag (channel/bridge).

Resilience fixes for the updater itself:

- Hourly checks and the boot check now run in SILENT mode: network
  errors don't promote to a red error state, they're logged and
  retried on the next tick. Only user-initiated "Check now" surfaces
  errors. This prevents the cascade of "Ошибка проверки" cards on
  flaky networks or partial uploads.
- Boot check retries up to 3 times (30s/2m/5m backoff) before giving
  up until the hourly tick.
- Track lastCheckedAt; "Up to date" subtitle now shows "checked Nm ago".

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/shared/types.ts

@@ -206,10 +206,10 @@ export const SAMPLE_EXERCISES: Omit<Exercise, 'id' | 'nextFireAt'>[] = [
 ]
 
 export type UpdaterStatus =
-  | { kind: 'idle' }
+  | { kind: 'idle'; lastCheckedAt?: number }
   | { kind: 'unsupported'; reason: string }
   | { kind: 'checking' }
-  | { kind: 'not-available'; currentVersion: string }
+  | { kind: 'not-available'; currentVersion: string; lastCheckedAt?: number }
   | { kind: 'available'; version: string; releaseDate?: string }
   | {
       kind: 'downloading'