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/clients.md

@@ -44,7 +44,7 @@ It ships as a real package, not just a source build — full steps in
 
 - **Any Flatpak distro (recommended)** — `flatpak install https://flatpak.unom.io/io.unom.Punktfunk.flatpakref`
   from the hosted [`flatpak.unom.io`](/docs/install-client#linux-desktop-flatpak) repo, then
-  `flatpak update`; this is also what the Decky plugin launches.
+  `flatpak update`; this is also what the [Decky plugin](/docs/steam-deck) launches.
 - **Ubuntu / Debian** — `apt install punktfunk-client` from the punktfunk apt registry.
 - **Fedora / Bazzite** — `rpm-ostree install punktfunk-client` from the Gitea RPM registry.
 - **Arch / SteamOS** — the `punktfunk-client` split package from the `PKGBUILD`.
@@ -108,7 +108,8 @@ punktfunk-probe --connect <host>:9777 --pin <fp>  # connect to one
 | You're streaming to… | Use |
 |---|---|
 | A Mac, iPhone, iPad, or Apple TV | The **Apple app** |
-| A Linux desktop or laptop, or a Steam Deck | **`punktfunk-client`** (GTK4) |
+| A Linux desktop or laptop | **`punktfunk-client`** (GTK4) |
+| A **Steam Deck** | The **[Decky plugin](/docs/steam-deck)** in Gaming Mode, or the GTK4 client in Desktop Mode |
 | An Android phone or TV | The **Android app** |
 | Windows | The native **`punktfunk-client`** (signed MSIX) or **Moonlight** |
 | A browser, a smart TV, or any other device | **Moonlight** |