Fix M3 capture: render format, project rate, unsaved-project handling

RENDER_FORMAT now uses the base64 float-WAV config (raw bytes were rejected
into a 16-bit/44.1 fallback); RENDER_SRATE pinned to the project rate.
Unsaved projects prompt Save then refuse if cancelled — no default-location
fallback. Harden deriveBankPaths against empty projectDir.

src/capture.h

@@ -70,6 +70,7 @@ enum class CaptureStatus {
     NoProject,          // no active project to render / resolve a bank folder
     EmptyRange,         // start >= end: nothing to render
     UnsupportedMode,    // backend does not implement this source mode (M3 scope)
+    UnsupportedFormat,  // requested bit depth has no known REAPER blob (M3: Float32 only)
     RenderFailed,       // the render action ran but produced no output file
 };