fix(filesharing): announce decrypt progress to AT, trim removal comment (#242)

Follow-up to #241.

- Wrap DecryptionProgress in `role="status" aria-live="polite"` so the
  Yivi → Decrypting transition is announced to assistive tech (WCAG
  4.1.3 Status Messages).
- Indeterminate progressbar: drop `aria-valuemin`/`aria-valuemax`
  (only meaningful alongside `aria-valuenow`) and add
  `aria-busy="true"`.
- Trim the multi-line removal-context comment in fallback/Decrypt.svelte
  to a single line stating the current invariant.

Closes #138

Co-authored-by: dobby-yivi-agent[bot] <275734547+dobby-yivi-agent[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: dobby-coder[bot]

src/lib/components/filesharing/DecryptionProgress.svelte

@@ -35,6 +35,7 @@
             class="bar-track"
             role="progressbar"
             aria-labelledby="decryption-progress-label"
+            aria-busy="true"
         >
             <div class="bar-indeterminate"></div>
         </div>