fix flatpak build: add checksums and update to GNOME 49

- Generate cargo checksum files for vendored sources
- Add checksums.json for build-time checksum creation
- Update runtime to GNOME 49 for Rust 1.93+ support
- Fix archive-type to tar-gzip for flatpak-builder

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

org.nextbike.NextCompanion.yml

@@ -1,6 +1,6 @@
 app-id: org.nextbike.NextCompanion
 runtime: org.gnome.Platform
-runtime-version: '47'
+runtime-version: '49'
 sdk: org.gnome.Sdk
 sdk-extensions:
   - org.freedesktop.Sdk.Extension.rust-stable
@@ -30,6 +30,16 @@ modules:
     build-commands:
       - mkdir -p .cargo
       - cp cargo-vendor-config/cargo-vendor-config.toml .cargo/config.toml
+      - |
+          python3 -c "
+          import json
+          with open('cargo-checksums/checksums.json') as f:
+              checksums = json.load(f)
+          for name, checksum in checksums.items():
+              path = f'cargo-vendor/{name}/.cargo-checksum.json'
+              with open(path, 'w') as f:
+                  json.dump({'files': {}, 'package': checksum}, f)
+          "
       - cargo --offline build --release
       - |
           install -Dm755 \
@@ -46,4 +56,8 @@ modules:
         path: build-aux/cargo-vendor-config.toml
         dest: cargo-vendor-config
         dest-filename: cargo-vendor-config.toml
+      - type: file
+        path: build-aux/cargo-sources-checksums.json
+        dest: cargo-checksums
+        dest-filename: checksums.json
       - build-aux/cargo-sources.json