fix: insecure fallback directory and secure file creation (#111)

* 🔒 Fix insecure fallback directory and secure file creation

The daemon's fallback runtime directory `get_runtime_dir()` was hardcoded to `/run/pwsp`, creating a risk of shared, insecure access in multi-user systems.
This commit secures the fallback logic by:
1. Creating a user-specific temporary directory (`/tmp/pwsp-$UID`).
2. Ensuring directory creation happens atomically with `0o700` permissions using `std::fs::DirBuilder`.
3. Validating the fallback directory strictly (checking UID, 0o700 permissions, and symlink status) if it already exists to mitigate symlink attacks.
4. Using `libc::geteuid()` for robust cross-platform UID extraction.
5. Fixing `is_daemon_running` and locking logic to use `fs::OpenOptions` instead of `fs::File::create` to prevent accidental file truncation on active lock files.

Co-authored-by: arabianq <55220741+arabianq@users.noreply.github.com>

* 🔒 Fix insecure fallback directory and secure file creation

The daemon's fallback runtime directory `get_runtime_dir()` was hardcoded to `/run/pwsp`, creating a risk of shared, insecure access in multi-user systems.
This commit secures the fallback logic by:
1. Creating a user-specific temporary directory (`/tmp/pwsp-$UID`).
2. Ensuring directory creation happens atomically with `0o700` permissions using `std::fs::DirBuilder`.
3. Validating the fallback directory strictly (checking UID, 0o700 permissions, and symlink status) if it already exists to mitigate symlink attacks.
4. Using safe `rustix::process::geteuid()` for robust cross-platform UID extraction, avoiding `unsafe` blocks.
5. Fixing `is_daemon_running` and locking logic to use `fs::OpenOptions` instead of `fs::File::create` to prevent accidental file truncation on active lock files.

Co-authored-by: arabianq <55220741+arabianq@users.noreply.github.com>

* small refactor

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

src/bin/daemon.rs

@@ -39,7 +39,11 @@ async fn main() -> Result<()> {
 
     let runtime_dir = get_runtime_dir();
 
-    let lock_file = fs::File::create(runtime_dir.join("daemon.lock"))?;
+    let lock_file = fs::OpenOptions::new()
+        .create(true)
+        .write(true)
+        .truncate(false)
+        .open(runtime_dir.join("daemon.lock"))?;
     lock_file.lock()?;
 
     let socket_path = runtime_dir.join("daemon.sock");