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>

clients/decky/bin/punktfunkrun.sh

@@ -31,4 +31,6 @@ fi
 echo "punktfunkrun: streaming $APPID --connect $PF_HOST" >&2
 # exec so the flatpak client IS the game process — when it exits, Steam ends the "game" and
 # Gaming Mode reclaims focus automatically (no manual refocus needed).
-exec "$FLATPAK" run --arch=x86_64 "$APPID" --connect "$PF_HOST"
+# --fullscreen: present the stream chrome-less and fullscreen (the client also auto-detects the
+# Deck/gamescope env, and ignores the flag harmlessly on older builds that predate it).
+exec "$FLATPAK" run --arch=x86_64 "$APPID" --connect "$PF_HOST" --fullscreen