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

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

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

Closes #190 Phase 1

lan-client/ui.js

@@ -1 +1 @@
-{ "placeholder": "will be replaced" }
+CONTENT_PLACEHOLDER