🔒 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>

Cargo.lock

@@ -3172,12 +3172,12 @@ dependencies = [
  "egui_material_icons",
  "evdev",
  "itertools 0.14.0",
- "libc",
  "opener",
  "pipewire",
  "rfd",
  "rodio",
  "rust-i18n",
+ "rustix 1.1.4",
  "serde",
  "serde_json",
  "sys-locale",

Cargo.toml

@@ -62,7 +62,7 @@ eframe = { version = "0.34.2", default-features = false, features = [
 egui_extras = "0.34.1"
 egui_material_icons = "0.6.0"
 egui_dnd = "0.15.0"
-libc = "0.2.186"
+rustix = { version = "1.1.4", features = ["process"] }
 
 [[bin]]
 name = "pwsp-daemon"

src/utils/daemon.rs

@@ -38,7 +38,7 @@ pub fn get_daemon_config() -> DaemonConfig {
 }
 
 fn get_current_uid() -> u32 {
-    unsafe { libc::geteuid() }
+    rustix::process::geteuid().as_raw()
 }
 
 pub fn get_runtime_dir() -> PathBuf {