feat(ui): hide GCG export in honest-AI games

A vs_ai game is throwaway practice, so its finished history header no longer
offers the 📤 GCG export (an empty slot keeps the comms icon pinned right).
Docs note the AI exclusion; UI_DESIGN also records that confirming a resign
reveals the full board (closes the history drawer, zooms out).

docs/UI_DESIGN.md

@@ -300,9 +300,11 @@ enabled on the first, uncached load) and flip in place when an event refreshes t
   Safari.
 - **History / GCG**: the in-game slide-down history lays each move out in a per-seat grid
   (the word(s) and the move score, no running total); *Export GCG* (the 📤 in the history
-  header) shares or downloads the `.gcg` file and appears only once the game is finished.
+  header) shares or downloads the `.gcg` file and appears only once the game is finished — and
+  never in an honest-AI game (throwaway practice). Confirming a resign reveals the full board:
+  it closes the history drawer (portrait) and zooms the board out.
 - **Finished game**: the board keeps no last-word highlight and no zoom; the history header
-  offers *Export GCG* (not *Drop game*) and the comms hub hides the 🔎 *Dictionary* tab; and
+  offers *Export GCG* (not *Drop game*; an AI game offers neither) and the comms hub hides the 🔎 *Dictionary* tab; and
   the footer (rack + tab bar) is **drawn but inert** (greyed, non-interactive) rather than
   hidden, so the layout does not jump. Chat send / nudge are the ⬆️ / 🛎️ icons. The input
   row is turn-driven: on your turn the message field shows until your one allowed message is