feat: add flatpak validation workflow

.github/workflows/validate-flatpaks.yml

@@ -0,0 +1,43 @@
+name: Validate Flatpaks
+permissions:
+  contents: read
+
+on:
+  pull_request:
+    paths:
+      - "flatpaks/**"
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
+
+      - name: Get Flathub apps
+        shell: bash
+        run: |
+          FLATHUB_LIST=$(mktemp)
+          curl -X 'GET' \
+            'https://flathub.org/api/v2/appstream?filter=apps' \
+            -H 'accept: application/json' | jq -r ".[]" >> "${FLATHUB_LIST}"
+
+          curl -X 'GET' \
+            'https://flathub.org/api/v2/appstream?filter=runtime' \
+            -H 'accept: application/json' | jq -r ".[]" >> "${FLATHUB_LIST}"
+
+          set -xeuo pipefail
+
+          find "flatpaks" -iname '*.list*' -print0 | \
+            while IFS= read -r -d '' flatpaks_file; do
+              echo "::group:: ===$(basename "$flatpaks_file")==="
+              grep -vE '^#' "$flatpaks_file" | while read -r flatpak; do
+                # Only output on errors
+                if ! grep -Fxq "$flatpak" "$FLATHUB_LIST"; then
+                  echo "$flatpak NOT found on Flathub"
+                  exit 1
+                fi
+              done
+
+              echo "::endgroup::"
+            done
+