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/install-client.md

@@ -16,7 +16,7 @@ Whichever client you install, the first connection needs a one-time [pairing](/d
 | Device | Install |
 |--------|---------|
 | **Linux** desktop / laptop | [Flatpak](#linux-desktop-flatpak) (any distro) or native apt/rpm/Arch packages |
-| **Steam Deck** | [Flatpak in Desktop Mode](#steam-deck) (or the Decky plugin) |
+| **Steam Deck** | [Decky plugin](/docs/steam-deck) for Gaming Mode, or [Flatpak in Desktop Mode](#steam-deck) |
 | **Windows** | [Signed MSIX](#windows) from the package registry |
 | **macOS** | [Notarized `.dmg`](#macos) from the releases page |
 | **iPhone / iPad / Apple TV** | [TestFlight beta](#ios-ipados-apple-tv) |
@@ -57,16 +57,23 @@ punktfunk-client --connect <host>:9777
 
 ## Steam Deck
 
-In **Desktop Mode**, install the Flatpak exactly as [above](#linux-desktop-flatpak) — it carries
-its own libadwaita + SDL3 and survives SteamOS updates:
+Most Deck users want **Gaming Mode**: install the **[Decky plugin](/docs/steam-deck)** and a
+**punktfunk** panel lands in the Quick Access Menu, so you can discover hosts, pair with a PIN, and
+stream **without dropping to the desktop**. Follow the **[Steam Deck (Decky) guide](/docs/steam-deck)**
+— it walks through Decky Loader, the plugin, and the one-time client install.
+
+> The plugin doesn't decode video itself — it launches the Flatpak client below. The Decky guide
+> covers installing both, so start there: a Flatpak on its own won't add the Gaming Mode panel.
+
+For **Desktop Mode** (or to add the client to Game Mode as a non-Steam app yourself), install the
+Flatpak exactly as [above](#linux-desktop-flatpak) — it carries its own libadwaita + SDL3 and
+survives SteamOS updates:
 
 ```sh
 flatpak install --user https://flatpak.unom.io/io.unom.Punktfunk.flatpakref
 ```
 
-Add it to Game Mode as a non-Steam app, or use the **Decky plugin**, which launches this same
-Flatpak (`flatpak run io.unom.Punktfunk --connect …`). See
-[packaging/flatpak](https://git.unom.io/unom/punktfunk/src/branch/main/packaging/flatpak/README.md).
+See [packaging/flatpak](https://git.unom.io/unom/punktfunk/src/branch/main/packaging/flatpak/README.md).
 
 ## Windows