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/frontend.nix

@@ -1,12 +1,21 @@
-{flutter}:
-flutter.buildFlutterApplication {
-  pname = "oott-frontend";
-  version = "0.1.0";
-  src = ./../frontend;
+{flutter}: let
+  # Single source of truth: extract the version from frontend/pubspec.yaml.
+  # Nix has no YAML parser, so split into lines and read the `version:` line.
+  # (Matching the whole file with a single regex is avoided on purpose: nested
+  # quantifiers over the full text trigger catastrophic backtracking in Nix's
+  # regex engine.)
+  lines = builtins.filter builtins.isString (builtins.split "\n" (builtins.readFile ./../frontend/pubspec.yaml));
+  versionLine = builtins.head (builtins.filter (line: builtins.match "version:.*" line != null) lines);
+  version = builtins.head (builtins.match "version: *([^ ]+) *" versionLine);
+in
+  flutter.buildFlutterApplication {
+    pname = "oott-frontend";
+    inherit version;
+    src = ./../frontend;
 
-  # Fetches pub dependencies from the committed pubspec.lock (all hosted, no git deps).
-  autoPubspecLock = ./../frontend/pubspec.lock;
+    # Fetches pub dependencies from the committed pubspec.lock (all hosted, no git deps).
+    autoPubspecLock = ./../frontend/pubspec.lock;
 
-  # Build the web bundle (the backend serves these assets).
-  targetFlutterPlatform = "web";
-}
+    # Build the web bundle (the backend serves these assets).
+    targetFlutterPlatform = "web";
+  }

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;