feat(ui): show build number in About dialog (v1.0.0-rc.8+<build>)

Add package_info_plus 8.3.1 (Flutter Community Plus, BSD-3, ~3M
downloads) and resolve the displayed version string from the
platform manifest at app init. The string shown in the About
dialog is now formatted as 'v<version>+<build>' (e.g.
'v1.0.0-rc.8+60') where both halves come from the SAME
pubspec.yaml 'version:' field that Flutter uses to populate iOS's
CFBundleShortVersionString + CFBundleVersion and Android's
versionName + versionCode.

Motivation: during the iOS VoiceProcessingIO audio rollout the
user is rebuilding repeatedly from Xcode. Without a build-number
suffix there is no way to confirm from inside the app which
commit produced the build under test — they all read
'v1.0.0-rc.8'. Every test commit going forward bumps the +<build>
field in pubspec.yaml so the About dialog uniquely identifies the
build.

Implementation:
* pubspec.yaml: version 1.0.0-rc.8+59 -> +60 (this commit is a
  test build). Add package_info_plus: ^8.0.0.
* main.dart: _kAppVersion changes from 'const String' to
  'String' (resolved at runtime). Populated in main() before
  runApp() via new _resolveAppVersion() helper that calls
  PackageInfo.fromPlatform() and formats 'v<info.version>+<info.buildNumber>'.
  Falls back to the hardcoded 'v1.0.0-rc.8' baseline if the
  platform call fails (extremely unlikely; the platform channel
  is a constant lookup).

The consumer site at the About dialog (l10n.aboutVersion(_kAppVersion))
is unchanged — the variable still resolves to a String. flutter
analyze: clean.

apps/chanora_flutter/pubspec.lock

@@ -309,6 +309,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.0.3"
+  http:
+    dependency: transitive
+    description:
+      name: http
+      sha256: "87721a4a50b19c7f1d49001e51409bddc46303966ce89a65af4f4e6004896412"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.6.0"
   http_multi_server:
     dependency: transitive
     description:
@@ -469,6 +477,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.2.0"
+  package_info_plus:
+    dependency: "direct main"
+    description:
+      name: package_info_plus
+      sha256: "16eee997588c60225bda0488b6dcfac69280a6b7a3cf02c741895dd370a02968"
+      url: "https://pub.dev"
+    source: hosted
+    version: "8.3.1"
+  package_info_plus_platform_interface:
+    dependency: transitive
+    description:
+      name: package_info_plus_platform_interface
+      sha256: "202a487f08836a592a6bd4f901ac69b3a8f146af552bbd14407b6b41e1c3f086"
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.2.1"
   path:
     dependency: transitive
     description:
@@ -730,6 +754,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.0.3"
+  win32:
+    dependency: transitive
+    description:
+      name: win32
+      sha256: d7cb55e04cd34096cd3a79b3330245f54cb96a370a1c27adb3c84b917de8b08e
+      url: "https://pub.dev"
+    source: hosted
+    version: "5.15.0"
   xdg_directories:
     dependency: transitive
     description: