Fixes batch 1: modal focus, campaign system, combat difficulty, origin desc

- Modal: focus into the dialog ONCE on open (not on every parent re-render), and
  prefer the first field over the X button — fixes focus jumping to the close
  button on each keystroke in the handout composer (and every other composer).
- Campaign edit: hide the System selector entirely (it's fixed after creation).
- Combat difficulty: rate against the PCs actually in the encounter (falls back to
  the roster) so adding a player updates the reading.
- Character builder: ancestry/background description is now a scrollable panel
  instead of clamped to 3 lines (no longer cut off on small screens).

223 unit + key e2e green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/features/characters/builder/CreationWizard.tsx

@@ -378,7 +378,7 @@ function OriginPicker({ title, options, value, onPick }: { title: string; option
         ))}
         {options.length === 0 && <p className="text-xs text-muted">Loading…</p>}
       </div>
-      {sel?.desc && <p className="mt-1 line-clamp-3 text-xs text-muted">{sel.desc}</p>}
+      {sel?.desc && <p className="mt-1 max-h-32 overflow-y-auto whitespace-pre-wrap rounded-md border border-line bg-surface-2 p-2 text-xs text-muted">{sel.desc}</p>}
     </div>
   );
 }