Fix:theme_blank plot margin

[hurry060215-tech]

Summary

• Increase the default theme_blank() plot margin from lab_size + 2 to lab_size + 10 on all sides.
• Reposition coordinate axis labels away from the origin: the x label sits just beyond the x-axis arrow, and the y label sits just beyond the y-axis arrow.
• Add a small testthat check that locks the default margin for coordinate annotations.

Why

theme_blank() draws coordinate arrows and axis labels outside the panel with coord_cartesian(clip = off). The previous default margin was too small for plots such as CellDimPlot(..., theme_use = theme_blank), so the coordinate annotation could be clipped at the canvas edge. The labels were also anchored at the coordinate origin, which made them visually touch the axes.

Validation

• Ran git diff --check successfully.
• Ran devtools::test(stop_on_failure = TRUE) successfully: PASS 1 / FAIL 0.
• Rendered a local preview with longer labels (Standardpca_1, Standardpca_2) to visually check the annotation placement.

[hurry060215-tech]

已重新整理 \ heme_blank()\ 的画布和坐标标注布局，最新提交 \6317e85\ 已推送到 PR 分支。主要调整：plot margin 提高到 \lab_size + 10\；x 轴标注放到 x 轴箭头右下侧；y 轴标注放到 y 轴箭头左上侧，避免原点附近重叠和裁剪。已跑 \devtools::test(stop_on_failure=TRUE)\，结果 PASS 1 / FAIL 0。