a11y: cut + merge progress bars role=progressbar + aria-valuenow

Following 4.6.64 for the queue progress bars, the cut + merge
progress containers in their respective tabs had the same gap:
a plain <div class="progress-bar"> wrapping a <div class="progress-
bar-fill"> with no semantic role. JS poked the bar's style.width
on every percent update; AT had no way to read out the running
value.

Promoted both .progress-bar wrappers to role="progressbar" with
aria-valuemin / max / now, plus aria-label sourced from new
locale strings (cutProgressAria / mergeProgressAria) so EN/DE
both work.

The progress event handlers in renderer.ts now also stamp
aria-valuenow on each tick, so AT live regions pick up the
percentage as the cut / merge advances. setAttribute is cheap
relative to the FFmpeg progress event rate (~1/s), no perf
concern.

renderer-texts.applyText sets the localized aria-label on both
gauges at boot + language switch — text contents already get
re-applied through the same path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/index.html

@@ -372,7 +372,7 @@
                         </div>
 
                         <div class="progress-container" id="cutProgress">
-                            <div class="progress-bar">
+                            <div class="progress-bar" role="progressbar" aria-valuemin="0" aria-valuemax="100" aria-valuenow="0" aria-label="Cut progress" id="cutProgressGauge">
                                 <div class="progress-bar-fill" id="cutProgressBar"></div>
                             </div>
                             <div class="progress-text" id="cutProgressText">0%</div>
@@ -404,7 +404,7 @@
                         </div>
 
                         <div class="progress-container" id="mergeProgress">
-                            <div class="progress-bar">
+                            <div class="progress-bar" role="progressbar" aria-valuemin="0" aria-valuemax="100" aria-valuenow="0" aria-label="Merge progress" id="mergeProgressGauge">
                                 <div class="progress-bar-fill" id="mergeProgressBar"></div>
                             </div>
                             <div class="progress-text" id="mergeProgressText">0%</div>