feat(ui): economics monitor insert panel with placeholder data (#824)

New implant panel at implant/economics: system selector, 6-commodity
price table with trend indicators, GDP strip. Composed from D-169
component library. Ring buffer caches last 20 ticks per system.
Snapshot routing wired through snapshot_handler → GameState →
snapshot_consumers → economics_panel. Placeholder prices shown
until server ships EconomySnapshot (#822).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/scripts/autoloads/hud_groups.gd

@@ -14,6 +14,7 @@ extends Node
 ##   "implant/map/starchart"   — star map navigator
 ##   "implant/wiki/gttr"       — Drifter's Guide reader
 ##   "implant/journal"         — knowledge journal
+##   "implant/economics"       — economics monitor (D-181, #824)
 ##
 ## Usage:
 ##   HudGroups.register(self, "implant/map/starchart")