fix: upload status announcement

[kristian-zendato]

Resolves

Fixes: https://github.com/nextcloud-gmbh/customer-feature-requests/issues/1622

Summary

• Add a stable polite aria-live status region on UploadPicker so screen readers announce upload start, throttled progress (~25% steps), completion, cancel.
• Expose progress via aria-valuetext (percent + optional total size) and tighten isUploading so the progress UI only stays visible for initialized / uploading / assembling items.

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

[codecov]

Codecov Report

❌ Patch coverage is 0% with 61 lines in your changes missing coverage. Please review.
✅ Project coverage is 33.01%. Comparing base (5b68676) to head (1b7033f).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/vue/components/UploadPicker.vue	0.00%	61 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2171      +/-   ##
==========================================
- Coverage   35.47%   33.01%   -2.46%     
==========================================
  Files          18       18              
  Lines         792      851      +59     
  Branches      150      166      +16     
==========================================
  Hits          281      281              
- Misses        498      557      +59     
  Partials       13       13              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.