fix: use cumulative CPU average for ffmpeg monitoring

Delta sampling over 2s returns 0-1 integer ticks for low-CPU processes
(ffmpeg copy mode ~0.3% CPU), always rounding to 0.0%. Switch to
cumulative average (total_cpu_ticks / clk_tck / elapsed) — same method
ps uses — which gives stable, accurate readings. Also show 2 decimal
places so values like 0.34% are visible.

frontend/src/pages/Dashboard.tsx

@@ -237,7 +237,7 @@ export default function Dashboard() {
         <StatCard
           icon={<Cpu size={20} className={totalCpu > 80 ? 'text-red-400' : totalCpu > 40 ? 'text-yellow-400' : 'text-cyan-400'} />}
           label="ffmpeg CPU total"
-          value={`${totalCpu.toFixed(1)}%`}
+          value={`${totalCpu.toFixed(2)}%`}
         />
       </div>
 
@@ -330,7 +330,7 @@ export default function Dashboard() {
                               s.ffmpeg_cpu_pct > 40 ? 'text-yellow-400' :
                               'text-gray-400'
                             }`}>
-                              {s.ffmpeg_cpu_pct.toFixed(1)}%
+                              {s.ffmpeg_cpu_pct.toFixed(2)}%
                             </span>
                           )}
                         </div>