Feature/candidate grid scroll

[antigoneum]

Add optional candidate grid scroll layout

Summary

This PR adds an optional candidate grid layout for Weasel. When enabled through style
configuration, candidates can be shown as a fixed-column matrix and navigated by
row, similar to the native macOS candidate panel scroll behavior.

Generated with Codex assistance.

Behavior

• Adds style/layout/grid to opt into grid rendering.
• Adds style/layout/grid_columns, grid_visible_rows,
  grid_cell_width, and grid_cell_height for matrix sizing.
• Defaults to a 5-column, 5-row visible candidate window.
• Uses +, =, and numpad + to move the active row downward.
• Uses - and numpad - to move the active row upward.
• Uses number keys 1..grid_columns to select the candidate in the current row.
• Scrolls the visible candidate window only after the active row moves past the
  5 visible rows.

Example configuration

patch:
  "menu/page_size": 25
  "style/horizontal": true
  "style/layout/type": horizontal
  "style/layout/grid": true
  "style/layout/grid_columns": 5
  "style/layout/grid_visible_rows": 5
  "style/layout/grid_cell_width": 62
  "style/layout/grid_cell_height": 27

Notes

The IPC style struct is extended with the grid fields, so all Weasel components
need to be rebuilt and installed from the same revision.

Test

• Formatted touched C++ files with the repository .clang-format
  (BasedOnStyle: Chromium, SortIncludes: false).
• Built local x64 WeaselUI and RimeWithWeasel targets on Windows with
  VS Build Tools.
• WeaselServer compilation passed dependency targets but final link was
  blocked locally because output/WeaselServer.exe was running.
• Verified grid rendering with a 5-column, 5-row visible candidate window.
• Verified row movement and candidate selection with +, -, and number keys.
• Verified DPI scaling for configured grid cell dimensions.

[fxliang]

看上去像是要实现所谓的卷轴功能，但是目前这样的实现其实太多硬编逻辑，后面要清理是很难的。

预期更优的路径应该是librime要支持这种模式，设定卷轴行列，然后在开启了这种模式的时候不是用现有的单页的逻辑取数据，librime中selector或翻页等操作应该因应这种设定做对应的处理调整变化，等等。这样前端才不用硬编按键响应，行列信息等代码。

暂时更多只能是观望状态，望知悉