ci(release): split canary/stable tracks + unified Gitea Releases

A push to main publishes canary builds to canary channels (fast iteration,
unchanged); a single vX.Y.Z tag releases every platform at one version to the
stable channels and attaches all artifacts (.deb/.rpm/.msix/.apk/.aab/.dmg +
flatpak/decky/host-installer) to one Gitea Release. Collapses the
host-v*/win-v*/host-win-v* tag namespaces into v* — the channel split makes the
version-shadow bug structurally impossible (canary and stable are separate repos,
never a shared version line).

- scripts/ci/gitea-release.{sh,ps1}: one idempotent release helper
  (create-or-fetch + delete-before-upload), replacing 3 copy-pasted inline blocks
  and fixing their latent 409-on-reupload bug; prerelease flag auto-derived from
  the tag (an -rc tag won't shadow "Latest")
- channels: apt canary/stable distributions; rpm *-canary/base groups; flatpak
  canary/stable OSTree branches + a 2nd .Canary.flatpakref; generic-registry
  canary/ vs latest/ aliases; Play internal/alpha; Apple TestFlight vs notarized DMG
- android versionName threaded through gradle (versionCode stays run_number);
  Apple canary = TestFlight-only (no DMG/tvOS); canary base bumped to 0.3.0
- docs: new docs-site channels.md (subscribe table + cut-a-release runbook +
  box migration), refreshed ci.md workflow table + packaging READMEs

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs-site/content/docs/install-client.md

@@ -7,6 +7,10 @@ This page is the **install path for each client device**. For what each client *
 pick, see [Clients](/docs/clients); to install the **host**, see [Install the Host](/docs/install).
 Whichever client you install, the first connection needs a one-time [pairing](/docs/pairing).
 
+> The links below are the **stable** channel (moves on `vX.Y.Z` releases). For the latest `main`
+> build, use the **canary** channel — TestFlight / Play Internal, the `…Canary.flatpakref`, or the
+> `canary/` download URLs. See [Release Channels](/docs/channels).
+
 ## Pick your device
 
 | Device | Install |