feat: integrate rclone WebDAV backup for PC-to-PC file sync

Source PC serves backups/ folder via rclone WebDAV server (auto-starts with app).
Receiver PC pulls backups on schedule using rclone sync.
Network Backup UI now has two tabs: Rclone and API Key.
Rclone installed automatically via postinstall script.

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

scripts/install-rclone.sh

@@ -0,0 +1,23 @@
+#!/bin/bash
+# Install rclone if not already present
+if command -v rclone &> /dev/null; then
+  echo "rclone is already installed: $(rclone version | head -1)"
+  exit 0
+fi
+
+echo "Installing rclone..."
+if command -v curl &> /dev/null; then
+  curl -s https://rclone.org/install.sh | sudo bash
+elif command -v wget &> /dev/null; then
+  wget -qO- https://rclone.org/install.sh | sudo bash
+else
+  echo "WARNING: Neither curl nor wget found. Please install rclone manually:"
+  echo "  https://rclone.org/install/"
+  exit 0
+fi
+
+if command -v rclone &> /dev/null; then
+  echo "rclone installed successfully: $(rclone version | head -1)"
+else
+  echo "WARNING: rclone installation may have failed. Please install manually."
+fi