feat(app,stream): storage and hardware-decoding settings

- the streaming cache's budget (1-16 GiB) is chosen in Settings, shows
  what it holds, and can be emptied; a lower budget evicts at once
- hardware decoding can be turned off for a driver that shows green
  frames, from the next file
- both live in desktop.json rather than the playback prefs shared with
  Android, whose bridge record would reset them on every save. A file
  that will not parse is never overwritten

crates/pstr-core/src/config.rs

@@ -73,6 +73,11 @@ impl AppDirs {
         Ok(())
     }
 
+    /// Settings only the desktop app has: hardware decoding, the cache budget.
+    pub fn desktop_prefs_file(&self) -> PathBuf {
+        self.config.join("desktop.json")
+    }
+
     /// The desktop window's size, for the next launch to open at.
     pub fn window_file(&self) -> PathBuf {
         self.config.join("window.json")