fix: resolve startup crash and CrashScreen button crashes (#127)

Two bugs caused the crash-at-startup report:

1. CrashScreen used the widget's build context (above its own MaterialApp)
   for ScaffoldMessenger.of() in button callbacks. When the screen is the
   root widget — the runApp() path after a startup crash — there is no
   ScaffoldMessenger above it, so both 'Copy to Clipboard' and 'Report Issue
   on Codeberg' crashed with a null check error. Fix: wrap Scaffold.body in
   Builder to obtain a context that is a descendant of the Scaffold.

2. path_provider_android 2.2.21 updated to Pigeon 26, which causes a
   channel-error on startup for some Android devices. Pin to <2.2.21
   (resolves to 2.2.20, which uses the stable pre-Pigeon-26 implementation).
   Additionally, make initDatabasePath() catch PlatformException so a
   channel error at the very start of main() no longer hard-crashes the app;
   _openConnection()'s lazy fallback retries after runApp() completes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pubspec.lock

@@ -731,10 +731,10 @@ packages:
     dependency: "direct overridden"
     description:
       name: path_provider_android
-      sha256: "149441ca6e4f38193b2e004c0ca6376a3d11f51fa5a77552d8bd4d2b0c0912ba"
+      sha256: e122c5ea805bb6773bb12ce667611265980940145be920cd09a4b0ec0285cb16
       url: "https://pub.dev"
     source: hosted
-    version: "2.2.23"
+    version: "2.2.20"
   path_provider_foundation:
     dependency: transitive
     description: