Fix: prevent wg0.conf truncation when remove_peer splits blocks

_write_config() was stripping trailing newlines, causing the next
add_cell_peer() to create a single-newline separator between [Interface]
and [Peer] blocks instead of the required blank line. On the following
remove_peer() call, split('\n\n') treated both sections as one block,
matched the PublicKey filter, and wrote an empty string — destroying the
[Interface] section and reverting to the hardcoded SERVER_ADDRESS fallback.

Two-part fix:
1. _write_config() always ends content with a newline
2. remove_peer() normalises single-newline [Peer] headers to blank-line
   separators before splitting, and refuses to write if [Interface] would
   be lost

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

api/wireguard_manager.py

@@ -294,6 +294,8 @@ class WireGuardManager(BaseServiceManager):
         return self.generate_config()
 
     def _write_config(self, content: str):
+        if content and not content.endswith('\n'):
+            content += '\n'
         with open(self._config_file(), 'w') as f:
             f.write(content)
         self._syncconf()
@@ -805,12 +807,20 @@ class WireGuardManager(BaseServiceManager):
         """Remove the [Peer] block matching public_key from wg0.conf."""
         try:
             content = self._read_config()
-            # Split on blank lines between blocks
+            # Normalise to ensure blank-line block separators before splitting.
-            raw_blocks = ('\n' + content).split('\n\n')
+            # Without this, a file written without trailing newline will merge
+            # [Interface] and the first [Peer] into one block, and the filter
+            # below would then delete [Interface] together with the peer.
+            normalised = content.replace('\n[Peer]', '\n\n[Peer]')
+            raw_blocks = ('\n' + normalised).split('\n\n')
             new_blocks = [
                 b for b in raw_blocks
                 if not (f'PublicKey = {public_key}' in b and '[Peer]' in b)
             ]
+            # Never write an empty file — that would destroy the [Interface] block.
+            if not any('[Interface]' in b for b in new_blocks):
+                logger.error('remove_peer: [Interface] block would be lost — aborting write')
+                return False
             self._write_config('\n\n'.join(new_blocks).lstrip('\n'))
             return True
         except Exception as e: