Derive the version from the Cargo/pubspec manifests

The version string was duplicated across six places. Collapse it to two
ecosystem sources of truth and derive the rest:

- backend/src/web_server.rs: omit the OpenAPI info.version so utoipa fills
  it from CARGO_PKG_VERSION (backend/Cargo.toml); add a test pinning this.
- nix/package.nix: read the version from backend/Cargo.toml via fromTOML.
- nix/frontend.nix: read the version from frontend/pubspec.yaml by splitting
  into lines (a whole-file regex triggers catastrophic backtracking in Nix's
  regex engine).
- frontend/lib/about/about.dart: read the version at runtime via
  package_info_plus instead of a hardcoded constant.

Also drop frontend/pubspec.lock.json: it is unreferenced (Nix's
autoPubspecLock generates its own JSON from pubspec.lock) and was going stale.

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

nix/package.nix

@@ -12,7 +12,8 @@
 in
   pkgs.rustPlatform.buildRustPackage rec {
     pname = "oott";
-    version = "0.1.0";
+    # Single source of truth: read the version straight from backend/Cargo.toml.
+    version = (builtins.fromTOML (builtins.readFile ./../backend/Cargo.toml)).package.version;
     src = ./../backend;
     cargoLock = {
       lockFile = ./../backend/Cargo.lock;