[IrisRainbowNeko]

Implements the request in #743, kept small and opt-in.

A color scheme can now draw an image behind the panel, as image themes do in Fcitx5 and Weasel skins. Three optional keys under preset_color_schemes/<name>:

back_image: skins/panel.png      # relative to the Rime user directory; ~ is expanded
back_image_slice: { left: 40, right: 60, top: 20, bottom: 30 }     # nine-slice, in image points
back_image_padding: { left: 30, right: 50, top: 16, bottom: 24 }   # room around the panel where only the image shows

• The corners of the slice keep their size, the edges and the center stretch (CALayer.contentsCenter). HiDPI images are drawn at their point size.
• The panel itself is unchanged and sits inside the padding; back_color: 0x00ffffff lets the image show under the candidates.
• The window is never smaller than the fixed corners; extra room goes to the right and bottom so the panel stays at the caret.
• The image stays upright in vertical mode, and is not used in full-screen mode.
• Schemes without back_image behave exactly as before: the image layer stays hidden and the padding is zero.

The change is small (3 files, +93/-11): two NSEdgeInsets and a CGImage in SquirrelTheme, and in SquirrelPanel a panelView that holds the existing views and is inset by the padding, with an image view under it.

Tested on GitHub arm64 runners — this branch on macOS 26, the same change backported to 1.1.2 on macOS 15 — by showing SquirrelPanel with a test image (colored corners of different sizes, striped center) in linear, stacked, vertical and single-candidate layouts and capturing the window. SwiftLint and Periphery report nothing new.

在配色方案中可用一张图片作候选窗背景（九宫格拉伸），对应 #743 的需求；不设 back_image 时行为不变。