fix(auth): persist PKCE state in sessionStorage

Store OIDC code_verifier and state in sessionStorage instead of
static memory variables. This fixes the "No code verifier" error
that occurred after Authentik redirect because the Flutter app
restarts and loses in-memory state.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

pubspec.yaml

@@ -16,7 +16,7 @@ publish_to: 'none' # Remove this line if you wish to publish to pub.dev
 # https://developer.apple.com/library/archive/documentation/General/Reference/InfoPlistKeyReference/Articles/CoreFoundationKeys.html
 # In Windows, build-name is used as the major, minor, and patch parts
 # of the product and file versions while build-number is used as the build suffix.
-version: 1.0.6+1
+version: 1.0.7+1
 
 environment:
   sdk: ^3.10.4