feat: add NexaNote app branding

[TheZupZup]

Summary

User-facing branding polish so installed builds present as NexaNote.

• Android launcher label: changed android:label from "app" to "NexaNote" in app/android/app/src/main/AndroidManifest.xml. The app drawer and installer now show "NexaNote" instead of the default "app".
• Launcher icon: replaced the default Flutter placeholder ic_launcher.png in all mipmap-* densities with a clean indigo-gradient "N" mark. Added an adaptive icon (mipmap-anydpi-v26/ic_launcher.xml + per-density ic_launcher_foreground.png + ic_launcher_background color in values/colors.xml) for API 26+.
• Docs: rewrote app/README.md (was the default Flutter template) with a branding section and APK build steps.

Package id (com.nexanote.app), pubspec name/version (nexanote 1.0.0+2), and the Linux window title (NexaNote) were already correct and left unchanged. No backend, sync, or storage logic touched. Total icon assets ~40K.

How to rebuild the APK

cd app
flutter pub get
flutter build apk --release

APK output: build/app/outputs/flutter-apk/app-release.apk, installs as NexaNote.

Test plan

• flutter pub get (Flutter SDK not available in this environment — not run here)
• flutter build apk --release
• Install APK and confirm app drawer label and launcher icon show NexaNote branding

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Generated by Claude Code