fix(canvas/FilesTab): add aria-modal="false" to inline alertdialog confirm prompts (WCAG 4.1.2)

The two FilesTab confirm dialogs (delete-all, delete-one) use role="alertdialog"
but were missing aria-modal. These are inline in-page prompts without focus
trapping — aria-modal="false" explicitly documents the non-modal nature so
assistive technology knows the rest of the page remains interactive.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

canvas/src/components/tabs/FilesTab.tsx

@@ -266,7 +266,7 @@ function PlatformOwnedFilesTab({
         // immediately. Delete-All hovers DARKER (bg-red-700) — same AA
         // contrast trap that bit ConfirmDialog/ApprovalBanner. Cancel
         // lifts to surface-elevated instead of the prior no-op hover.
-        <div role="alertdialog" aria-labelledby="files-delete-all-msg" className="mx-3 mt-2 px-3 py-2 bg-red-950/30 border border-red-800/40 rounded space-y-1.5">
+        <div role="alertdialog" aria-modal="false" aria-labelledby="files-delete-all-msg" className="mx-3 mt-2 px-3 py-2 bg-red-950/30 border border-red-800/40 rounded space-y-1.5">
           <p id="files-delete-all-msg" className="text-xs text-bad">Delete all {files.filter((f) => !f.dir).length} files? This cannot be undone.</p>
           <div className="flex gap-2">
             <button type="button" onClick={() => { handleDeleteAll(); setShowDeleteAll(false); }} className="px-2 py-0.5 bg-red-700 hover:bg-red-600 text-[10px] rounded text-white transition-colors focus:outline-none focus-visible:ring-2 focus-visible:ring-red-500/60 focus-visible:ring-offset-1 focus-visible:ring-offset-surface">Delete All</button>
@@ -280,7 +280,7 @@ function PlatformOwnedFilesTab({
       )}
 
       {confirmDelete && (
-        <div role="alertdialog" aria-labelledby="files-delete-one-msg" className="mx-3 mt-2 px-3 py-2 bg-amber-950/30 border border-amber-800/40 rounded space-y-1.5">
+        <div role="alertdialog" aria-modal="false" aria-labelledby="files-delete-one-msg" className="mx-3 mt-2 px-3 py-2 bg-amber-950/30 border border-amber-800/40 rounded space-y-1.5">
           <p id="files-delete-one-msg" className="text-xs text-warm">Delete <span className="font-mono">{confirmDelete}</span>{files.find((f) => f.path === confirmDelete && f.dir) ? " and all its contents" : ""}?</p>
           <div className="flex gap-2">
             <button type="button" onClick={confirmDeleteFile} className="px-2 py-0.5 bg-red-700 hover:bg-red-600 text-[10px] rounded text-white transition-colors focus:outline-none focus-visible:ring-2 focus-visible:ring-red-500/60 focus-visible:ring-offset-1 focus-visible:ring-offset-surface">Delete</button>