fix(web): hide Retry when blob unavailable; failed-state surfaces actionable error

Pending bubbles that survive a reload (or post-redeploy refresh) without a
FileSystemFileHandle had no way to recover the original File bytes, so the
Retry button silently no-op'd: resumeUpload set state back to 'paused' with
no surfaced error, leaving the user stuck.

- transferStore: add reactive hasInMemoryFile Set mirroring liveUploadFiles.
  setInMemoryRef helper keeps both in sync at every set/delete site
  (startUpload, resumeUpload, onSuccess, remove). Persisted shape is unchanged.
- resumeUpload: when no blob is reachable (no in-memory File and no FS handle),
  call setError with an actionable "File no longer available — discard and
  re-upload" message instead of silently flipping back to 'paused'.
- Message.tsx: compute canRetry reactively from transfersForRow + hasInMemoryFile;
  hide the Retry button when retry is infeasible. Discard remains.
- AttachmentProgress: optional error prop surfaces transfer.error.message via
  title= on the failed-state ring for hover context.
- Tests: existing 3 resumeUpload tests now assert state==='failed' with the
  actionable message; +2 new tests for hasInMemoryFile lifecycle (start/remove,
  abort retains).

packages/web/src/components/chat/AttachmentProgress.tsx

@@ -6,6 +6,8 @@ interface Props {
   total: number;
   state: TransferState;
   filename: string;
+  /** Optional human-readable error surfaced as a hover tooltip when state==='failed'. */
+  error?: string;
   onPause?: () => void;
   onResume?: () => void;
   onAbort?: () => void;
@@ -19,7 +21,7 @@ function fmt(bytes: number): string {
   return `${(bytes / 1024 / 1024 / 1024).toFixed(2)} GB`;
 }
 
-export function AttachmentProgress({ loaded, total, state, filename, onPause, onResume, onAbort, size = 'tile' }: Props) {
+export function AttachmentProgress({ loaded, total, state, filename, error, onPause, onResume, onAbort, size = 'tile' }: Props) {
   const pct = total > 0 ? Math.min(100, Math.round((loaded / total) * 100)) : 0;
   const bg = state === 'failed' ? 'bg-accent-rose/30' : 'bg-accent-mint/30';
   const isFinal = state === 'completed' || state === 'aborted';
@@ -28,6 +30,7 @@ export function AttachmentProgress({ loaded, total, state, filename, onPause, on
       <div
         className={`w-9 h-9 rounded-full ${bg} flex items-center justify-center`}
         style={state !== 'failed' ? { background: `conic-gradient(rgba(180,220,200,.85) ${pct}%, rgba(255,255,255,.15) ${pct}%)` } : undefined}
+        title={state === 'failed' ? error : undefined}
       >
         <div className="w-7 h-7 rounded-full bg-surface-overlay text-[10px] text-txt-primary flex items-center justify-center font-medium">
           {state === 'failed' ? '!' : `${pct}%`}