fix(frontend): floating dropdowns and matrix modal frame use bg-paper

MitreTechniquePicker dropdown, SimulationList overflow menu,
ExportEngagementButton format menu, and MitreMatrixModal dialog frame
all used bg-canvas as their surface color. With the tinted canvas
(#f3f5f8), these floating surfaces appeared slightly grey instead of
clean white. Switched to bg-paper (#ffffff light / #1f2937 dark).
MitreMatrixModal cell hover (bg-canvas) intentionally preserved —
matrix cells sit on canvas, not on paper.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/MitreMatrixModal.tsx

@@ -170,7 +170,7 @@ export function MitreMatrixModal({
         role="dialog"
         aria-modal="true"
         aria-labelledby="matrix-modal-title"
-        className="relative bg-canvas rounded-none border border-hairline max-w-[98vw] max-h-[80vh] overflow-hidden flex flex-col"
+        className="relative bg-paper rounded-none border border-hairline max-w-[98vw] max-h-[80vh] overflow-hidden flex flex-col"
         style={{ width: '1400px' }}
         onKeyDown={handleKeyDown}
       >