fix: prevent multiple instances from running simultaneously (#245)

* fix: prevent multiple instances from running simultaneously

Add single-instance enforcement using file locking to prevent duplicate
instances when auto-start is enabled on boot (issue #244).

Changes:
- Add single_instance.py module with fcntl-based file locking
- Check for existing instance early in main() before any initialization
- Register atexit handler to release lock on normal exit
- Lock file located at ~/.local/share/vocalinux/instance.lock

If another instance is already running, the app exits gracefully with
a helpful error message telling the user how to resolve the situation.

* fix: add GTK notification when duplicate instance detected

Show a system notification to the user when another instance is already
running, in addition to the existing stderr/log output.

Changes:
- Import Notify from gi.repository when duplicate instance detected
- Show notification with error icon and message
- Give notification time to display before exiting (0.5s sleep)
- Fallback to silent exit if notification fails (e.g., headless)

src/vocalinux/single_instance.py

@@ -0,0 +1,98 @@
+"""
+Single instance enforcement for Vocalinux.
+
+Uses file locking to ensure only one instance runs at a time.
+Works across sessions and handles stale locks from crashed processes.
+"""
+
+import fcntl
+import logging
+import os
+import sys
+from pathlib import Path
+from typing import Optional
+
+logger = logging.getLogger(__name__)
+
+# Lock file location: ~/.local/share/vocalinux/
+LOCK_FILE_DIR = Path.home() / ".local" / "share" / "vocalinux"
+LOCK_FILE_PATH = LOCK_FILE_DIR / "instance.lock"
+
+# Global lock file handle
+_lock_file: Optional[int] = None
+
+
+def _get_lock_file_fd() -> int:
+    """Get or create the lock file descriptor."""
+    global _lock_file
+
+    if _lock_file is not None:
+        return _lock_file
+
+    # Ensure directory exists
+    LOCK_FILE_DIR.mkdir(parents=True, exist_ok=True)
+
+    # Open lock file (create if doesn't exist)
+    # O_RDWR: read/write, O_CREAT: create if not exists
+    fd = os.open(str(LOCK_FILE_PATH), os.O_RDWR | os.O_CREAT, 0o644)
+    _lock_file = fd
+
+    return fd
+
+
+def acquire_lock() -> bool:
+    """
+    Acquire the single instance lock.
+
+    Returns:
+        True if lock acquired (first instance), False if another instance is running.
+
+    If another instance is running, logs a message and returns False.
+    """
+    try:
+        fd = _get_lock_file_fd()
+
+        # Try to acquire exclusive lock (non-blocking)
+        # LOCK_EX: exclusive lock
+        # LOCK_NB: non-blocking (fail immediately if lock held by another process)
+        try:
+            fcntl.flock(fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
+        except (OSError, IOError):
+            # Lock is held by another process
+            logger.warning("Another instance of Vocalinux is already running. Exiting.")
+            print("Another instance of Vocalinux is already running.", file=sys.stderr)
+            print(
+                f"If you're sure no other instance is running, delete: {LOCK_FILE_PATH}",
+                file=sys.stderr,
+            )
+            return False
+
+        # Lock acquired - write our PID for debugging purposes
+        os.ftruncate(fd, 0)
+        os.write(fd, f"{os.getpid()}\n".encode())
+        os.lseek(fd, 0, os.SEEK_SET)
+
+        logger.info("Single instance lock acquired")
+        return True
+
+    except Exception as e:
+        # Unexpected error - log and allow startup (fail open but warn)
+        logger.error(f"Error acquiring single instance lock: {e}")
+        return True
+
+
+def release_lock():
+    """Release the single instance lock."""
+    global _lock_file
+
+    if _lock_file is not None:
+        try:
+            # Release lock
+            fcntl.flock(_lock_file, fcntl.LOCK_UN)
+            os.close(_lock_file)
+            _lock_file = None
+            logger.debug("Single instance lock released")
+        except (OSError, IOError) as e:
+            logger.warning(f"Failed to release lock: {e}")
+            # Ensure clean state
+            _lock_file = None