fix(flatpak): move runtime to KDE 6.10 (freedesktop 25.08)

Restore the Flatpak manifest to the repo and target org.kde.Platform
6.10 instead of the EOL 6.6/23.08 stack reported in issue #5.

Add a maintainer build script and a CI job that verifies the manifest
builds against the current KDE SDK.

scripts/build-flatpak.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+root="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+cd "$root"
+
+flatpak-builder --repo=flatpak_repo --force-clean flatpak_build de.nerdbear.bearwave.json
+flatpak build-update-repo flatpak_repo
+
+echo "Flatpak repo updated in flatpak_repo/. Deploy it to flatpak.bearwave.app."