feat(decky): self-update without the store + Gaming-Mode launch polish, and ship the Steam Deck docs

Plugin self-update (no Decky store): CI publishes a per-channel manifest.json
({version, immutable per-version artifact, sha256}) beside the zip and bakes
update.json {channel, manifest} into the plugin. main.py `check_update` reads the
installed version from package.json (the value Decky reports — not plugin.json),
fetches the channel manifest, and the frontend shows an "Update to vX" button that
drives Decky Loader's own install RPC (root downloads + SHA-256-verifies + hot-reloads).
CI now stamps a plain-numeric semver (0.3.<run> canary / X.Y.Z stable) into
package.json — a -ciN suffix would mis-order under compare-versions.

Linux client: `--fullscreen` (plus SteamDeck/gamescope env fallback) enters GTK
fullscreen on stream start so Gaming-Mode chrome is hidden; native-mode resolution
falls back to the display's first monitor when the window isn't mapped yet (was
dropping to the 1080p floor — wrong on the Deck's 1280×800); add a confirmed
"Remove saved host" action (KnownHosts::remove_by_fp).

Docs: new docs/steam-deck.md (Decky install/pair/stream/self-update/troubleshooting),
wired into meta.json nav, and cross-linked from clients/install-client/channels. This
is the page docs.punktfunk.unom.io/docs/steam-deck — the website's download link
pointed at it before it existed; committing it makes that link resolve.

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

docs-site/content/docs/channels.md

@@ -44,9 +44,10 @@ one-line edit of `/etc/apt/sources.list.d/punktfunk.list` (`stable` ↔ `canary`
 
 1. Make sure `main` is green.
 2. (Optional) bump any user-facing version that isn't derived from the tag — the Android
-   `versionName` fallback (`clients/android/app/build.gradle.kts`) and the Decky `plugin.json`
-   `version` are cosmetic self-reported strings; everything else (binaries via
-   `PUNKTFUNK_BUILD_VERSION`, MSIX, apt/rpm, the `.dmg`) derives from the tag automatically.
+   `versionName` fallback (`clients/android/app/build.gradle.kts`) is a cosmetic self-reported
+   string; everything else (binaries via `PUNKTFUNK_BUILD_VERSION`, MSIX, apt/rpm, the `.dmg`, and
+   the **Decky** plugin version — CI stamps it into `package.json`, where it drives the plugin's own
+   [self-update check](/docs/steam-deck#updating)) derives from the tag automatically.
 3. Tag and push — **one** tag releases every platform:
    ```sh
    git tag v0.2.0