feat(update): add version check to skip update when already latest

Now `nupst update` checks current version against latest release before
downloading anything.

Behavior:
- Fetches latest version from Gitea API
- Compares with current version
- Shows "Already up to date!" if versions match
- Only downloads/installs if newer version available

Example output when up to date:
  Checking for updates...
  Current version: v4.0.8
  Latest version:  v4.0.8

  ✓ Already up to date!

ts/cli/service-handler.ts

@@ -133,22 +133,43 @@ export class ServiceHandler {
       );
 
       console.log('');
-      logger.info('Updating NUPST to latest version...');
-      console.log('');
+      logger.info('Checking for updates...');
 
       try {
+        // Get current version
+        const currentVersion = this.nupst.getVersion();
+
+        // Fetch latest version from Gitea API
+        const apiUrl = 'https://code.foss.global/api/v1/repos/serve.zone/nupst/releases/latest';
+        const response = execSync(`curl -sSL ${apiUrl}`).toString();
+        const release = JSON.parse(response);
+        const latestVersion = release.tag_name; // e.g., "v4.0.7"
+
+        logger.dim(`Current version: ${currentVersion}`);
+        logger.dim(`Latest version:  ${latestVersion}`);
+        console.log('');
+
+        // Compare versions (both are in format "v4.0.7")
+        if (currentVersion === latestVersion) {
+          logger.success('Already up to date!');
+          console.log('');
+          return;
+        }
+
+        logger.info(`New version available: ${latestVersion}`);
+        logger.dim('Downloading and installing...');
+        console.log('');
+
         // Download and run the install script
         // This handles everything: download binary, stop service, replace, restart
         const installUrl = 'https://code.foss.global/serve.zone/nupst/raw/branch/main/install.sh';
 
-        logger.dim('Downloading install script...');
         execSync(`curl -sSL ${installUrl} | bash`, {
           stdio: 'inherit', // Show install script output to user
         });
 
         console.log('');
-        logger.success('Update completed successfully!');
-        logger.dim('Run "nupst service status" to verify the update.');
+        logger.success(`Updated to ${latestVersion}`);
         console.log('');
       } catch (error) {
         console.log('');