Flatpak: build from source instead of repackaging debs

- Rewrite Flatpak manifest to build from source using SDK extensions
  (node22, rust-stable) with offline cargo/npm dependency caches
- Add generate-sources.sh to produce cargo-sources.json and
  node-sources.json from lock files using flatpak-builder-tools
- Vendor Node.js, protoc, and wasm-pack as archive sources with
  SHA256 verification and per-arch support
- Add SHA256 verification to vendor-node.cjs and vendor-protoc.cjs
- Rewrite update-manifest.sh to update git tag/commit and regenerate
  dependency sources
- Update Flathub CI workflow to install generators and copy source
  files
- Remove stale plugins/faker entry from package-lock.json

.gitignore

@@ -48,3 +48,6 @@ crates-tauri/yaak-app/tauri.worktree.conf.json
 # Flatpak build artifacts
 flatpak-repo/
 .flatpak-builder/
+flatpak/flatpak-builder-tools/
+flatpak/cargo-sources.json
+flatpak/node-sources.json