feat: implement Openbox-style bottom window handle and grips

Add full handle/grip assembly to the bottom of SSD window frames,
following the Openbox themerc specification for geometry and theming.
Theme parsing:
- Parse window.handle.width (handle bar height, default 6)
- Parse window.grip.width (corner grip width, default 20)
- Parse window.[active|inactive].handle.bg with Solid/Gradient support
- Parse window.[active|inactive].grip.bg (inherits from handle if unset)
- Pre-render 1px-wide fill buffers and cairo patterns for handle/grip
Scene graph (new ssd-handle.c):
- Handle assembly replaces bottom border when active, with its own
  left/right/top borders and three-segment bottom border
- Grips at left/right corners for diagonal resize (sw/se-resize)
- Center handle for vertical resize (s-resize)
- Vertical separator lines between grips and handle using border color
- Per Openbox spec, handle_width is content-only height with borders
  drawn around it (total assembly height = 2*border_width + handle_width)
Interactive visual states (grips only):
- Hover: 20% black overlay on grip content area
- Pressed: 40% black overlay with 1px inset shadow (dark top/left,
  light bottom/right) for a pushed-in 3D effect
- Dragging: 20% overlay with inset shadow maintained
- Global hover tracking (server.hovered_handle_ssd/element) ensures
  proper cleanup when cursor moves across views or to desktop
Decoration toggle cycle (ToggleDecorations action):
- New LAB_SSD_MODE_BORDER_HANDLE between BORDER and FULL
- keepBorder=true: full -> border+handle -> border -> none -> full
- keepBorder=false: full -> none -> full (unchanged)
Node types and input:
- New LAB_NODE_HANDLE, LAB_NODE_GRIP_LEFT, LAB_NODE_GRIP_RIGHT
- Integrated into LAB_NODE_BORDER/BORDER_BOTTOM containment so
  existing Border context mousebinds (Resize) work automatically
- Handle/grip descriptors resolved directly in get_cursor_context()
  bypassing ssd_get_resizing_type() for precise cursor shapes
Visibility rules:
- Hidden when maximized, shaded, or handle_width is 0
- Hidden in LAB_SSD_MODE_BORDER and LAB_SSD_MODE_NONE states
- Bottom border in ssd-border.c disabled when handle is active
Documentation:
- labwc-theme.5.scd: document all handle/grip theme properties
- labwc-actions.5.scd: update ToggleDecorations to 4-state cycle
- docs/themerc: add handle/grip default values

include/labwc.h

@@ -229,6 +229,10 @@ struct server {
 
 	struct ssd_button *hovered_button;
 
+	/* Track which handle/grip element is hovered (for cross-view cleanup) */
+	struct ssd *hovered_handle_ssd;
+	int hovered_handle_element; /* -1 = none */
+
 	/* Tree for all non-layer xdg/xwayland-shell surfaces */
 	struct wlr_scene_tree *workspace_tree;