Initial O3DE Flatpak packaging + Gitea CI

Repackage the official O3DE Linux .deb as a Flatpak and publish it as a
static OSTree repo on the 'pages' branch for install via flatpak remote.

- org.o3de.O3DE.yaml: flatpak-builder manifest (extracts the .deb into /app)
- o3de-wrapper.sh: launcher that locates the versioned o3de binary at runtime
- desktop + AppStream metadata under the app-id
- scripts/: live version resolver and local build helper
- .gitea/workflows/build-flatpak.yml: daily check -> build -> publish -> tag

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/build.sh

@@ -0,0 +1,41 @@
+#!/usr/bin/env bash
+# Build the O3DE Flatpak locally (for testing the manifest before relying on CI).
+#
+# Requires: flatpak, flatpak-builder, curl, and the Flathub remote.
+# Note: O3DE is large (~15-18 GB installed); expect a multi-GB download and a
+# build that needs a lot of free disk space.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+eval "$(scripts/get-latest-version.sh)"
+echo ">> Latest O3DE: ${version} (${deb_file})"
+
+if [ ! -f o3de.deb ]; then
+  echo ">> Downloading ${deb_url}"
+  curl -fL --progress-bar -o o3de.deb "${deb_url}"
+fi
+
+if [ -n "${sha256:-}" ]; then
+  echo ">> Verifying checksum"
+  echo "${sha256}  o3de.deb" | sha256sum -c -
+else
+  echo ">> No checksum published; skipping verification" >&2
+fi
+
+# Make sure the runtime/SDK are available (no-op if already installed).
+flatpak remote-add --user --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
+flatpak install --user -y flathub org.freedesktop.Platform//24.08 org.freedesktop.Sdk//24.08 || true
+
+echo ">> Building"
+flatpak-builder --user --force-clean --install-deps-from=flathub \
+  --repo=repo build-dir org.o3de.O3DE.yaml
+
+cat <<EOF
+
+>> Done.
+   Test run without installing:
+     flatpak-builder --run build-dir org.o3de.O3DE.yaml o3de-wrapper.sh
+   Or install from the local repo:
+     flatpak remote-add --user --no-gpg-verify o3de-local repo
+     flatpak install --user o3de-local org.o3de.O3DE
+EOF