fix: audit findings — security, i18n, validation, dead code (v0.3.2)

Quality:
- Q-5: Allow relative session commands (e.g. niri-session), greetd resolves PATH
- Q-3: Socket read+write timeouts with proper error logging
- Q-2: Remove unused PowerError::Timeout variant
- Q-M1: i18n for all login_worker error messages (new: unexpected_greetd_response)
- Q-M2: Explicit INVALID_LIST_POSITION check in session dropdown
- Q-M4: Log SVG loader.close() errors instead of silencing
- Q-M6: Testable persistence functions with proper roundtrip tests

Security:
- S-2: Validate GTK theme name (alphanumeric, _, -, +, . only)
- S-3: Log file created with mode 0o640
- S-4: Cache files (last-user, last-session) created with mode 0o600

Performance:
- P-3: Single symlink_metadata() call instead of exists() + is_symlink()
- P-4: Avoid Vec allocation in IPC send_message (two write_all calls)

Config:
- Update example GTK theme to Colloid-Catppuccin

src/main.rs

@@ -88,9 +88,11 @@ fn setup_logging() {
     let log_dir = PathBuf::from("/var/cache/moongreet");
     if log_dir.is_dir() {
         let log_file = log_dir.join("moongreet.log");
+        use std::os::unix::fs::OpenOptionsExt;
         if let Ok(file) = std::fs::OpenOptions::new()
             .create(true)
             .append(true)
+            .mode(0o640)
             .open(&log_file)
         {
             builder.target(env_logger::Target::Pipe(Box::new(file)));