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>

scripts/upload-release-assets.ps1

@@ -1,22 +1,31 @@
 <#
 .SYNOPSIS
-  Upload pre-built NSIS artifacts to an existing Gitea release.
+  Upload pre-built NSIS artifacts to a Gitea release.
 
 .DESCRIPTION
-  Use when the tag v* is already pushed (e.g. release.ps1 succeeded up to
-  push but failed on upload, or release was created manually without assets).
-  If a release for the tag does not exist yet, it is created. If it exists,
-  same-name assets are replaced.
+  Uploads installer + blockmap + latest.yml to the release identified by -Tag.
+  If the release does not exist it is created (only for semver-looking tags;
+  for non-semver tags like 'update-channel' the release must exist already).
+  Same-named existing assets are replaced.
 
 .PARAMETER Tag
-  Version tag, e.g. v0.3.0. Defaults to v<package.json version>.
+  Release tag to upload INTO. May be a version tag (v0.5.1) or a channel
+  tag (update-channel). Defaults to v<package.json version>.
+
+.PARAMETER AssetVersion
+  Version of the artifacts being uploaded (e.g. 0.5.1). Defaults to the
+  numeric part of -Tag. Specify explicitly when uploading version-X.Y.Z
+  artifacts into a non-version tag (channel or bridge).
 
 .EXAMPLE
   pwsh scripts/upload-release-assets.ps1
-  pwsh scripts/upload-release-assets.ps1 -Tag v0.3.0
+  pwsh scripts/upload-release-assets.ps1 -Tag v0.5.0
+  pwsh scripts/upload-release-assets.ps1 -Tag update-channel -AssetVersion 0.5.1
+  pwsh scripts/upload-release-assets.ps1 -Tag v0.4.0 -AssetVersion 0.5.1
 #>
 param(
-  [string]$Tag
+  [string]$Tag,
+  [string]$AssetVersion
 )
 
 $ErrorActionPreference = 'Stop'
@@ -35,10 +44,18 @@ $root = Resolve-Path (Join-Path $PSScriptRoot '..')
 Set-Location $root
 
 if (-not $Tag) {
-  $version = (Get-Content package.json | ConvertFrom-Json).version
-  $Tag = "v$version"
+  $pkgVersion = (Get-Content package.json | ConvertFrom-Json).version
+  $Tag = "v$pkgVersion"
 }
-$version = $Tag.TrimStart('v')
+if (-not $AssetVersion) {
+  # Derive from tag when possible (vX.Y.Z → X.Y.Z); otherwise read package.json.
+  if ($Tag -match '^v\d+\.\d+\.\d+') {
+    $AssetVersion = $Tag.TrimStart('v')
+  } else {
+    $AssetVersion = (Get-Content package.json | ConvertFrom-Json).version
+  }
+}
+$version = $AssetVersion
 
 $installer = Join-Path 'release' "Exercise-Reminder-Setup-$version.exe"
 $blockmap  = "$installer.blockmap"
@@ -66,6 +83,10 @@ try {
   Write-Host "  Found release id=$($release.id)" -ForegroundColor DarkGray
 } catch {
   if ($_.Exception.Response.StatusCode.value__ -eq 404) {
+    if ($Tag -notmatch '^v\d+\.\d+\.\d+') {
+      Write-Error "Release '$Tag' not found and tag is not semver. Create it manually on Gitea (e.g. 'update-channel' is a one-time setup)."
+      exit 1
+    }
     Write-Host "  Not found, creating new release..." -ForegroundColor DarkGray
 
     $prev = $null