fix: panic with clear message if XDG data dir is unavailable

The previous fallback to PathBuf::from(".") silently placed the
database in whatever the current working directory was, making it
impossible to find later. Since the daemon cannot function without a
persistent database, failing fast with an actionable message is the
correct behavior.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

strata-daemon/src/config.rs

@@ -8,7 +8,10 @@ pub struct Config {
 impl Config {
     pub fn new() -> Self {
         let data_dir = dirs::data_dir()
-            .unwrap_or_else(|| PathBuf::from("."))
+            .expect(
+                "Could not determine XDG data directory. \
+                 Ensure $XDG_DATA_HOME or $HOME is set.",
+            )
             .join("strata");
 
         if let Err(e) = std::fs::create_dir_all(&data_dir) {