Add configurable buttons, separate settings screens and backup activity

- Add ButtonConfigActivity for customizing main screen buttons with
  drag-and-drop reordering and individual size options (S/M/L)
- Move storage settings to separate StorageSettingsActivity
- Move signature setting to storage settings (relevant for WebDAV sync)
- Move data backup to separate BackupActivity with export/import
- Make "more" overflow button configurable in size
- Simplify SettingsActivity to 3 navigation buttons
- Add logbook rename/delete functionality
- Improve S/M/L button contrast with visible borders

app/src/main/res/layout/activity_button_config.xml

@@ -0,0 +1,56 @@
+<?xml version="1.0" encoding="utf-8"?>
+<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:app="http://schemas.android.com/apk/res-auto"
+    android:layout_width="match_parent"
+    android:layout_height="match_parent"
+    android:orientation="vertical"
+    android:padding="20dp">
+
+    <TextView
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:text="@string/button_config_title"
+        android:textSize="28sp"
+        android:textColor="@color/accent"/>
+
+    <TextView
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:layout_marginTop="10dp"
+        android:text="@string/button_config_hint"
+        android:textColor="@android:color/darker_gray"/>
+
+    <androidx.recyclerview.widget.RecyclerView
+        android:id="@+id/button_list"
+        android:layout_width="match_parent"
+        android:layout_height="0dp"
+        android:layout_weight="1"
+        android:layout_marginTop="20dp"/>
+
+    <LinearLayout
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:layout_marginTop="20dp"
+        android:orientation="horizontal"
+        android:gravity="end">
+
+        <Button
+            android:id="@+id/btn_cancel"
+            android:layout_width="wrap_content"
+            android:layout_height="wrap_content"
+            android:layout_marginEnd="10dp"
+            android:background="@drawable/button_background"
+            android:textColor="@android:color/darker_gray"
+            android:text="@string/button_config_cancel"/>
+
+        <Button
+            android:id="@+id/btn_save"
+            android:layout_width="wrap_content"
+            android:layout_height="wrap_content"
+            android:background="@drawable/button_background"
+            android:textColor="@color/accent"
+            android:text="@string/button_config_save"/>
+
+    </LinearLayout>
+
+</LinearLayout>