the folder name is served, not derived three times

Mobile asked for `/targets` to carry each person's display name as the folder will
be named, so a picker can honestly say where files will appear. Right request: the
writer already computes that exact string to create a directory, and any client
computing its own would eventually disagree with it — which means telling somebody
the wrong place to look.

So `displayNameFor` is the one definition and the writer calls it too. `/targets`
gains `displayName` per person, and `myFolder` for the whole response: files land
in the RECIPIENT's tree under the SENDER's name, so that is one value rather than
a field on each row. The two are easy to confuse and the distinction matters — a
picker showing `displayName` as the destination folder would name the wrong one.

The web panel now uses both instead of its own copy of the fallback.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/servers/api/sendoff/paths.ts

@@ -49,3 +49,24 @@ export function safeFilename(name: string): string {
     .slice(0, 200);
   return cleaned || 'file';
 }
+
+/**
+ * The name an account appears as, and therefore the name of the folder its files land in.
+ *
+ * One definition, used by the writer AND by `/targets`, because the two must agree: a picker that says
+ * "Fedra" while the writer creates `fedra.almeida` has told somebody the wrong place to look. Mobile asked
+ * for this to be served rather than derived, which is right — three clients reimplementing a fallback is
+ * how three clients disagree.
+ *
+ * `users.name` is nullable, so the fallback chain ends at the account id, which is always there.
+ */
+export function displayNameFor(account: { id: number; name: string | null; email: string }): string {
+  const chosen = account.name?.trim();
+  if (chosen) return chosen;
+  const local = account.email.split('@')[0]?.trim();
+  return local || `user-${account.id}`;
+}
+
+/** What that name becomes on disk — the same value, through the segment sanitiser. */
+export const folderNameFor = (account: { id: number; name: string | null; email: string }): string =>
+  safeSegment(displayNameFor(account)) || `user-${account.id}`;