Anchor the update channel tag, and stop shipping a duplicate AppImage

Two defects in the update channel, both visible in 0.4.20 and 0.4.21.

**The channel tag does not survive.** `publish-update-channel.sh` created the
GitHub release, uploaded both assets and verified each URL returned 200 — the
job log shows it succeeding at 00:38. By 13:04 the tag was gone and every
installed copy was checking a 404.

Gitea push-mirrors this repo to GitHub every four hours, and a mirror push
deletes remote refs with no local counterpart. `linux-latest` was created by
GitHub's release API and never existed as a Gitea tag, so the mirror removed
it. Versioned tags were never affected because `create-tag` creates them in
Gitea first.

So the tag is now anchored in Gitea, and before the GitHub release rather than
after, so there is no window where the two disagree. Its absence fails the
step instead of warning, because it is the only thing keeping the channel
alive. Worth stating plainly: publishing correctly is not evidence the channel
still works, and the verification that passed at 00:38 could not have caught a
failure that arrives twelve hours later.

**Every release carried the AppImage twice.** The channel's stable-named copy
sat beside the versioned one, where the release job's `*.AppImage` glob picked
it up — so v0.4.21 published `Triple-C_0.4.21_amd64.AppImage` and
`Triple-C_x86_64.AppImage`, byte-identical at 86,686,200 bytes each, and
`sync-to-github` copied both to the mirror. 80 MB of duplicate per release,
under a name that reads like a different build. That is how it was noticed.

The channel pair now lives in `bundle/appimage/update-channel/`, out of the
glob's reach, and a guard fails the build if more than one AppImage is left
beside the release. Verified by planting a second one: it fails.

One appimagetool quirk found while moving it — zsyncmake writes the .zsync
into the working directory, not beside the image it describes, so it has to be
collected rather than assumed in place. The existing guard caught that too.

Verified against the real 0.4.19 artifact: exactly one AppImage at top level,
the channel pair in its own directory, update string still resolving to the
fixed tag, and the wayland fallback intact.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011YPqHpjV4EL6RNEwrRKqQm

.gitea/workflows/build-app-preview.yml

@@ -335,7 +335,6 @@ jobs:
         run: |
           mkdir -p artifacts
           cp app/src-tauri/target/release/bundle/appimage/*.AppImage artifacts/ 2>/dev/null || true
-          cp app/src-tauri/target/release/bundle/appimage/*.zsync artifacts/ 2>/dev/null || true
           ls -la artifacts/
 
       # Assets, not workflow artifacts — see the note at the top of this file.