🔒 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>

Cargo.lock

@@ -3172,6 +3172,7 @@ dependencies = [
  "egui_material_icons",
  "evdev",
  "itertools 0.14.0",
+ "libc",
  "opener",
  "pipewire",
  "rfd",