Workbench bill editor — tap a workbench, see/edit bills

Tap-to-select chain extended to workbenches (pawn always wins on shared
tile). Mutually exclusive with pawn selection via EventBus —
selecting one clears the other.

New WorkbenchPanel (scenes/ui/workbench_panel.gd, ~432 LOC, layer 18,
right-anchored 360 px) mirrors PawnDetailPanel shape. Bill rows expose
recipe name, mode (FOREVER / COUNT / UNTIL_N), target count, completed
progress, pause, and remove. Add-bill popup filters RecipeCatalog.all()
by accepted_skill so a Hearth only offers cooking recipes.

Supporting plumbing:
- EventBus.workbench_selected / workbench_deselected signals.
- Workbench.remove_bill() — interrupts mid-craft cleanly via
  on_craft_interrupted() before erasing.
- RecipeCatalog.all() static enumerator + Recipe.display_name() helper.
- World.workbench_at_tile() lookup.
- i18n keys ui.bill.* and ui.workbench.* in strings.gd.

Closes the deferred Phase 17 "Bill UI for workbenches" item. Player-
built workbenches are now functionally configurable; before this
landed, only world.gd-hardcoded bills worked.

scenes/ai/recipe.gd

@@ -46,6 +46,15 @@ var label: String = ""
 
 # ── save / load ───────────────────────────────────────────────────────────────
 
+## Player-visible display name for this recipe.  Used by the workbench bill
+## editor's recipe-picker and the bill list.  Falls back to `id` if `label`
+## is empty (shouldn't happen for catalog recipes, but defensive).
+func display_name() -> String:
+	if label.is_empty():
+		return str(id)
+	return label
+
+
 func to_dict() -> Dictionary:
 	return {
 		"id": String(id),