feat(phase-1): no-op mana pool display functions in ui.js

- renderManaPool() -> stub (floating #mana-pool-display removed from UI)
- showManaPool() -> stub
- Remove 'Mana Auto-Clear' row from renderSettingsPanel()

Internal mana tracking state (manaPool object, canPayManaCost,
clearManaPool, clearAllManaPools, etc.) fully preserved in engine.js.
createInitialState() already had energy/experience/rad/emblems.
setupManaPoolDrag() in engine.js already a no-op stub.
index.html and style.css require no changes.

lan-client/ui.js

@@ -1 +1 @@
-CONTENT_PLACEHOLDER
+CONTENT_FROM_VARIABLE