a11y: update-banner progress bar role=progressbar + aria-valuenow

Third progress bar in this a11y pass — the download-progress bar
shown in the update banner during an auto-update download. Same
pattern as 4.6.64 (queue) + 4.6.65 (cut/merge): bare div with
JS-driven width, no semantic role.

Promoted the .update-banner-progress-track to role="progressbar"
with aria-valuemin / max / now + a localized aria-label
(updateProgressAria: "Update download progress" / "Update-Download-
Fortschritt").

Three call sites in renderer-updates.ts that drive bar.style.width
now also stamp aria-valuenow on the gauge:
- onUpdateProgress event handler (per-tick percent)
- setDownloadPendingUi (initial 30% indeterminate placeholder)
- setDownloadReadyUi (100% on finish)

renderer-texts.applyText sets the localized aria-label at boot +
on language switch.

That's all three application-level progress bars now AT-friendly.
The same pattern would extend to any future progress UI.

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

src/index.html

@@ -11,7 +11,7 @@
     <div class="update-banner" id="updateBanner">
         <span id="updateText">Neue Version verfügbar!</span>
         <div id="updateProgress" class="update-banner-progress-wrap" style="display: none;">
-            <div class="update-banner-progress-track">
+            <div class="update-banner-progress-track" role="progressbar" aria-valuemin="0" aria-valuemax="100" aria-valuenow="0" aria-label="Update download" id="updateProgressGauge">
                 <div id="updateProgressBar" class="update-banner-progress-bar"></div>
             </div>
         </div>