v1.0.26: fix multi-selection reactivity, redesign icon, security review

Fix multi-selection: selectedKeys exposed as StateFlow, collected in
FilesScreen so checkboxes and highlights update correctly on every tap.
fileKey() made public so UI can check membership without ViewModel calls.

Icon: white cloud body with two cyan/teal circular sync arcs (AutoSync
style), deep blue-to-teal gradient background.

Security review clean: no hardcoded credentials, cleartext blocked by
network_security_config, allowBackup=false, path traversal guards in
place on both server responses and local resolution.

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

app/src/main/res/drawable/ic_launcher_background.xml

@@ -1,10 +1,36 @@
 <?xml version="1.0" encoding="utf-8"?>
-<shape xmlns:android="http://schemas.android.com/apk/res/android">
-    <gradient
-        android:type="radial"
-        android:gradientRadius="80%"
-        android:centerX="0.5"
-        android:centerY="0.4"
-        android:startColor="#1B2A3B"
-        android:endColor="#06090E"/>
-</shape>
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:aapt="http://schemas.android.com/aapt"
+    android:width="108dp"
+    android:height="108dp"
+    android:viewportWidth="108"
+    android:viewportHeight="108">
+
+    <!-- Deep blue-to-teal gradient background matching reference icon -->
+    <path
+        android:pathData="M0,0 H108 V108 H0 Z">
+        <aapt:attr name="android:fillColor">
+            <gradient
+                android:type="linear"
+                android:startX="0" android:startY="0"
+                android:endX="108" android:endY="108"
+                android:startColor="#1565C0"
+                android:endColor="#00897B"/>
+        </aapt:attr>
+    </path>
+
+    <!-- Subtle radial highlight in upper-right -->
+    <path
+        android:pathData="M108,0 A90,90 0 0,1 108,90 Z"
+        android:fillAlpha="0.18">
+        <aapt:attr name="android:fillColor">
+            <gradient
+                android:type="radial"
+                android:gradientRadius="80"
+                android:centerX="85" android:centerY="23"
+                android:startColor="#80DEEA"
+                android:endColor="#00000000"/>
+        </aapt:attr>
+    </path>
+
+</vector>