Implement Phase 1 Android client: Flutter share receiver + upload service

- Register ACTION_SEND IntentFilter for image/* in AndroidManifest
- Kotlin MethodChannel bridge: extract image URI, original filename, resolve to file
- Flutter bottom sheet UI: thumbnail preview, note input (200 char), submit/cancel
- Upload service: multipart POST, Bearer auth, 10MB limit, offline cache, exponential backoff retry
- Configurable server URL and API token via SharedPreferences
- Minimal home screen with usage instructions

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

windows/flutter/generated_plugin_registrant.cc

@@ -0,0 +1,11 @@
+//
+//  Generated file. Do not edit.
+//
+
+// clang-format off
+
+#include "generated_plugin_registrant.h"
+
+
+void RegisterPlugins(flutter::PluginRegistry* registry) {
+}