feat: add per-account lock file protection via fs2

Prevent running multiple tele-tui instances with the same account by
using advisory file locks (flock). Lock is acquired before raw mode so
errors print to normal terminal. Account switching acquires new lock
before releasing old. Also log set_tdlib_parameters errors via tracing
instead of silently discarding them.

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

src/accounts/lock.rs

@@ -0,0 +1,127 @@
+//! Per-account advisory file locking to prevent concurrent access.
+//!
+//! Uses `flock` (via `fs2`) for automatic lock release on process crash/SIGKILL.
+//! Lock file: `~/.local/share/tele-tui/accounts/{name}/tele-tui.lock`
+
+use fs2::FileExt;
+use std::fs::{self, File};
+use std::path::PathBuf;
+
+/// Returns the lock file path for a given account.
+///
+/// Path: `{data_dir}/tele-tui/accounts/{name}/tele-tui.lock`
+pub fn account_lock_path(account_name: &str) -> PathBuf {
+    let mut path = dirs::data_dir().unwrap_or_else(|| PathBuf::from("."));
+    path.push("tele-tui");
+    path.push("accounts");
+    path.push(account_name);
+    path.push("tele-tui.lock");
+    path
+}
+
+/// Acquires an exclusive advisory lock for the given account.
+///
+/// Creates the lock file and parent directories if needed.
+/// Returns the open `File` handle — the lock is held as long as this handle exists.
+///
+/// # Errors
+///
+/// Returns an error message if the lock is already held by another process
+/// or if the lock file cannot be created.
+pub fn acquire_lock(account_name: &str) -> Result<File, String> {
+    let lock_path = account_lock_path(account_name);
+
+    // Ensure parent directory exists
+    if let Some(parent) = lock_path.parent() {
+        fs::create_dir_all(parent).map_err(|e| {
+            format!(
+                "Не удалось создать директорию для lock-файла: {}",
+                e
+            )
+        })?;
+    }
+
+    let file = File::create(&lock_path).map_err(|e| {
+        format!("Не удалось создать lock-файл {}: {}", lock_path.display(), e)
+    })?;
+
+    file.try_lock_exclusive().map_err(|_| {
+        format!(
+            "Аккаунт '{}' уже используется другим экземпляром tele-tui.\n\
+             Lock-файл: {}",
+            account_name,
+            lock_path.display()
+        )
+    })?;
+
+    Ok(file)
+}
+
+/// Explicitly releases the lock by unlocking and dropping the file handle.
+///
+/// Used during account switching to release the old account's lock
+/// before acquiring the new one.
+pub fn release_lock(lock_file: File) {
+    let _ = lock_file.unlock();
+    drop(lock_file);
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_lock_path_structure() {
+        let path = account_lock_path("default");
+        let path_str = path.to_string_lossy();
+        assert!(path_str.contains("tele-tui"));
+        assert!(path_str.contains("accounts"));
+        assert!(path_str.contains("default"));
+        assert!(path_str.ends_with("tele-tui.lock"));
+    }
+
+    #[test]
+    fn test_lock_path_per_account() {
+        let path1 = account_lock_path("work");
+        let path2 = account_lock_path("personal");
+        assert_ne!(path1, path2);
+        assert!(path1.to_string_lossy().contains("work"));
+        assert!(path2.to_string_lossy().contains("personal"));
+    }
+
+    #[test]
+    fn test_acquire_and_release() {
+        let name = "test-lock-acquire-release";
+        let lock = acquire_lock(name).expect("first acquire should succeed");
+
+        // Second acquire should fail (same process, exclusive lock)
+        let result = acquire_lock(name);
+        assert!(result.is_err(), "second acquire should fail");
+        assert!(
+            result.unwrap_err().contains("уже используется"),
+            "error should mention already in use"
+        );
+
+        // Release and re-acquire
+        release_lock(lock);
+        let lock2 = acquire_lock(name).expect("acquire after release should succeed");
+
+        // Cleanup
+        release_lock(lock2);
+        let _ = fs::remove_file(account_lock_path(name));
+    }
+
+    #[test]
+    fn test_lock_released_on_drop() {
+        let name = "test-lock-drop";
+        {
+            let _lock = acquire_lock(name).expect("acquire should succeed");
+            // _lock dropped here
+        }
+
+        // After drop, lock should be free
+        let lock = acquire_lock(name).expect("acquire after drop should succeed");
+        release_lock(lock);
+        let _ = fs::remove_file(account_lock_path(name));
+    }
+}