fix(translate): M1-M3 medium + L1-L2 low bugs from QA review

M1: ReDoS guard — 500 char max + try/except on re.compile
M2: Performance note — O(n×m) regex documented for future optimization
M3: Verified test key names correct (metrics.py transforms cumulative_* → short)
L1: Removed @staticmethod no-op from module-level function
L2: Added aria-label to emoji indicators in CorrectionCell

frontend/src/lib/components/translate/CorrectionCell.svelte

@@ -183,11 +183,12 @@
 {:else if uxState === 'saved'}
 	<div class="flex items-center gap-2">
 		<span class="text-sm text-gray-700">{displayValue}</span>
-		<span class="text-xs text-green-600" title="Saved">📝</span>
+		<span class="text-xs text-green-600" title="Saved" aria-label="Edited" role="img">📝</span>
 		<button
 			onclick={openDictPopup}
 			class="ml-1 px-1.5 py-0.5 text-xs bg-amber-100 text-amber-700 rounded hover:bg-amber-200 transition-colors"
 			title="Submit to Dictionary"
+			aria-label="Submit to Dictionary"
 		>
 			📕 Dictionary
 		</button>
@@ -204,7 +205,7 @@
 {:else if uxState === 'dict_submitted'}
 	<div class="flex items-center gap-2">
 		<span class="text-sm text-gray-700">{displayValue}</span>
-		<span class="text-xs text-green-600 font-medium" title="Saved to dictionary">✅ Dictionary ✓</span>
+		<span class="text-xs text-green-600 font-medium" title="Saved to dictionary" aria-label="Submitted to dictionary" role="img">✅ Dictionary ✓</span>
 	</div>
 
 <!-- Error mode -->