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

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

Author: dobby-coder[bot]

src/lib/components/fallback/Decrypt.svelte

@@ -104,11 +104,8 @@
                 },
             })
 
-            // pg-js auto-unwraps `data:`-mode payloads (single-entry zip
-            // with `data.bin`) into DecryptDataResult.plaintext. Email
-            // fallback uploads are always `data:` mode, so this branch is
-            // the expected one. If we ever receive a multi-file result
-            // here, fall back to the first entry's bytes.
+            // Email fallback uploads are `data:` mode → plaintext;
+            // `files[0]` is a defensive fallback for unexpected shapes.
             const plaintext =
                 result.plaintext ??
                 new Uint8Array(await result.files[0].blob.arrayBuffer())

src/lib/components/filesharing/DecryptionProgress.svelte

@@ -10,7 +10,7 @@
     let determinate = $derived(typeof percentage === 'number')
 </script>
 
-<div class="container">
+<div class="container" role="status" aria-live="polite">
     <p class="label" id="decryption-progress-label">
         {$_('filesharing.decryptpanel.downloadingAndDecrypting')}
     </p>
@@ -35,8 +35,7 @@
             class="bar-track"
             role="progressbar"
             aria-labelledby="decryption-progress-label"
-            aria-valuemin="0"
-            aria-valuemax="100"
+            aria-busy="true"
         >
             <div class="bar-indeterminate"></div>
         </div>