Render Queue: resolve a bare project filename against the working directory

`resolve_output` takes the project's folder from `Path::new(path).parent()`.
For a project saved under a bare filename that is the empty path, which the
following `filter` discarded, so a saved project fell through to the
unsaved-project default and relative outputs went to ~/Movies (the user profile
folder on Windows) instead of next to the project.

    --project proj.ecproj      ~/Movies/clip.mp4
    --project ./proj.ecproj    ./clip.mp4

Both spellings name a project in the working directory, so both now resolve
there, as the doc comment on `resolve_output` already describes.

Closes #1427

crates/engine/src/render_queue.rs

@@ -362,8 +362,15 @@ impl Session {
         // Settings ▸ Export ▸ Default Output Folder, else the project's folder.
         let folder = self.prefs.export.default_output_folder.trim();
         let base = (!folder.is_empty()).then(|| std::path::PathBuf::from(folder));
-        let base =
-            base.or_else(|| self.path.as_deref().and_then(|s| std::path::Path::new(s).parent().map(|d| d.to_path_buf())).filter(|d| !d.as_os_str().is_empty()));
+        // A project saved under a bare filename has the empty path as its parent. It sits in the
+        // working directory, so resolve against that rather than falling through to the
+        // unsaved-project default below (#1427).
+        let base = base.or_else(|| {
+            self.path
+                .as_deref()
+                .and_then(|s| std::path::Path::new(s).parent())
+                .map(|d| if d.as_os_str().is_empty() { std::path::PathBuf::from(".") } else { d.to_path_buf() })
+        });
         // Unsaved project: ~/Movies, else the home folder (as FilmCraft's exports do). The working directory
         // is `/` for an app started from the macOS Finder, where nothing can be written (#446).
         let home = std::env::var_os("HOME").or_else(|| std::env::var_os("USERPROFILE")).map(std::path::PathBuf::from);