Rename app to Nikki, add about section with logo and author info

- App name: Memos App → Nikki (from Japanese 日記, diary)
- Theme: Theme.MemosApp → Theme.Nikki
- Composable: MemosAppTheme → NikkiTheme
- Backup filename: memos-backup.json → nikki-backup.json
- Settings about section: 96dp logo, app name, version,
  one-liner summary, author, and GitHub issues link
- Notification subtext fallback: "nikki"

Signed-off-by: Avinal Kumar <avinal.xlvii@gmail.com>
Co-Authored-By: Claude Opus 4.6 (1M context)

androidApp/src/main/AndroidManifest.xml

@@ -18,12 +18,12 @@
         android:label="@string/app_name"
         android:roundIcon="@mipmap/ic_launcher_round"
         android:supportsRtl="true"
-        android:theme="@style/Theme.MemosApp">
+        android:theme="@style/Theme.Nikki">
 
         <activity
             android:name=".MainActivity"
             android:exported="true"
-            android:theme="@style/Theme.MemosApp">
+            android:theme="@style/Theme.Nikki">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
                 <category android:name="android.intent.category.LAUNCHER" />

androidApp/src/main/res/values-night/themes.xml

@@ -1,5 +1,5 @@
 <resources>
-    <style name="Theme.MemosApp" parent="android:Theme.Material.Light.NoActionBar">
+    <style name="Theme.Nikki" parent="android:Theme.Material.Light.NoActionBar">
         <item name="android:statusBarColor">@android:color/transparent</item>
         <item name="android:navigationBarColor">@android:color/transparent</item>
     </style>

androidApp/src/main/res/values/strings.xml

@@ -1,3 +1,3 @@
 <resources>
-    <string name="app_name">Memos App</string>
+    <string name="app_name">Nikki</string>
 </resources>

androidApp/src/main/res/values/themes.xml

@@ -1,5 +1,5 @@
 <resources>
-    <style name="Theme.MemosApp" parent="android:Theme.Material.Light.NoActionBar">
+    <style name="Theme.Nikki" parent="android:Theme.Material.Light.NoActionBar">
         <item name="android:statusBarColor">@android:color/transparent</item>
         <item name="android:navigationBarColor">@android:color/transparent</item>
     </style>