UI pass: medieval-warm Theme + real Build drawer thumbnails

Theme (scenes/ui/medieval_theme.gd):
- Procedural builder for an app-wide Theme: parchment buttons on
  dark-wood border, tan panels, ink text, gold focus ring.
- Applied on the root Window in Main._ready, with a tree-walk that
  re-seeds every CanvasLayer's topmost Control (CanvasLayer
  interrupts the root-Window theme cascade in Godot 4).
- Late-mounting popups + modals get themed via a child_entered_tree
  hook on each CanvasLayer.

Build drawer thumbnails (scenes/ui/build_drawer_thumb.gd):
- New BuildDrawerThumb Control that dispatches on tool_id and draws
  a recognisable silhouette of the entity each tool builds. 17 tools
  covered: chop/mine/dig_grave/no_roof (Designate), stone+wood walls,
  wood+stone floors, door, crate, bed, torch, 5 workbenches
  (Carpenter/Smelter/Millstone/Hearth/Cremation Pyre), stockpile,
  graveyard.
- Replaces the flat ColorRect placeholder. _add_tool_btn signature
  changed from (label, color, callback) to (label, tool_id, callback).

scenes/main/main.gd

@@ -20,6 +20,11 @@ const RESUME_TOAST_SCRIPT: Script          = preload("res://scenes/ui/resume_toa
 # Phase 17 — PawnDetailPanel (layer 18) and SettingsMenu (layer 26).
 const PAWN_DETAIL_PANEL_SCRIPT: Script     = preload("res://scenes/ui/pawn_detail_panel.gd")
 const WORKBENCH_PANEL_SCRIPT: Script       = preload("res://scenes/ui/workbench_panel.gd")
+const MEDIEVAL_THEME_SCRIPT: Script        = preload("res://scenes/ui/medieval_theme.gd")
+
+# Built once in _ready and re-applied to any CanvasLayer-rooted Control because
+# CanvasLayer doesn't propagate the root-Window theme cascade.
+var _app_theme: Theme = null
 const SETTINGS_MENU_SCRIPT: Script         = preload("res://scenes/ui/settings_menu.gd")
 # Phase 17 (Agent B) — BuildDrawer bottom-sheet (layer 16).
 const BUILD_DRAWER_SCRIPT: Script          = preload("res://scenes/ui/build_drawer.gd")
@@ -40,6 +45,13 @@ func _ready() -> void:
 	assert(SaveSystem != null, "SaveSystem autoload missing")
 	assert(Autosave != null, "Autosave autoload missing")
 
+	# Medieval-warm Theme — assigned on the root Window first.  Cascade alone
+	# doesn't reach Controls inside CanvasLayers (CanvasLayer has no theme
+	# property), so we also walk the tree post-mount and apply to every Control
+	# encountered.  (2026-05-16 polish pass.)
+	_app_theme = MEDIEVAL_THEME_SCRIPT.build()
+	get_tree().root.theme = _app_theme
+
 	# Phase 15 — Storyteller UI layers. Runtime-instantiated so no .tscn edit is
 	# needed. CanvasLayer ensures correct draw order above World/TopBar regardless
 	# of parent-node position.
@@ -156,3 +168,33 @@ func _ready() -> void:
 			top_bar._add_work_log_btns()
 
 	Audit.log("main", "Phase 17 (Agent C) — WorkPriorityMatrix + AlertsLog mounted.")
+
+	# Apply the medieval theme to every Control under each CanvasLayer.
+	# CanvasLayers interrupt the root-Window theme cascade so we have to seed
+	# each one explicitly.  Defer one frame so panels that build their UI in
+	# _ready (PawnDetailPanel, WorkbenchPanel, BuildDrawer) finish first.
+	call_deferred("_apply_theme_to_canvas_layers")
+
+
+## Walks the scene tree and assigns _app_theme to every Control directly under
+## a CanvasLayer (the topmost Control in each layer's branch).  From there the
+## standard Control-to-Control cascade carries the theme to all descendants.
+## Also catches popups and modals that mount later via child_entered_tree.
+func _apply_theme_to_canvas_layers() -> void:
+	for c in get_children():
+		if c is CanvasLayer:
+			_seed_layer_theme(c)
+			# Watch for late additions (popup menus, modals).
+			if not c.child_entered_tree.is_connected(_on_layer_child_added):
+				c.child_entered_tree.connect(_on_layer_child_added)
+
+
+func _seed_layer_theme(layer: CanvasLayer) -> void:
+	for c in layer.get_children():
+		if c is Control and c.theme == null:
+			c.theme = _app_theme
+
+
+func _on_layer_child_added(node: Node) -> void:
+	if node is Control and node.theme == null:
+		node.theme = _app_theme