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/MitreTechniquePicker.tsx

@@ -107,7 +107,7 @@ export function MitreTechniquePicker({
       />
 
       {open && (
-        <div className="absolute z-20 w-full mt-xxs bg-canvas border border-steel rounded-none overflow-hidden">
+        <div className="absolute z-20 w-full mt-xxs bg-paper border border-steel rounded-none overflow-hidden">
           {isFetching && (
             <div className="px-md py-sm text-[14px] text-graphite">Searching…</div>
           )}