fix: apply_config bootstraps wg0.conf when file is empty

If wg0.conf exists but is empty or has no [Interface] section,
apply_config previously found no lines to update and silently
returned with no changes — leaving the container broken on next
restart with an empty config.

Fix: detect empty/missing [Interface] section and regenerate the
full config from generate_config() before applying field updates.

This was the root cause of port changes not propagating:
apply_config was called but found nothing to patch in an empty file.

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

api/wireguard_manager.py

@@ -216,7 +216,16 @@ class WireGuardManager(BaseServiceManager):
             return {'restarted': restarted, 'warnings': warnings}
         try:
             with open(cf) as f:
-                lines = f.readlines()
+                raw = f.read()
+
+            # Bootstrap from generate_config() if file is empty or has no [Interface]
+            if not raw.strip() or '[Interface]' not in raw:
+                raw = self.generate_config()
+                with open(cf, 'w') as f:
+                    f.write(raw)
+                warnings.append('wg0.conf was empty — regenerated from keys')
+
+            lines = raw.splitlines(keepends=True)
 
             def _set_iface_field(lines, key, value):
                 result = []