feat(canvas): unify visual styling and layout spacing

[nmrtist]

Summary

Establish a unified visual and layout system for the PlotX canvas.

Canvas editing chrome previously used scattered fixed colors, including
premultiplied translucent fills that could bleach underlying figures. Grid
gutters also described frame spacing only, so axis furniture could make the
visible separation between adjacent data areas much larger than expected.

This PR:

• Adds a theme-derived ChromeStyle role table for canvas editing feedback.
• Adds an optional canvas accent under Preferences → Appearance.
• Fixes translucent chrome to use unmultiplied alpha.
• Redesigns tile previews with subtle fills and differentiated outlines.
• Shows independent dashed guides for non-zero page margins.
• Adds independent axis-title and tick-label visibility controls.
• Keeps axis visibility consistent across screen, SVG, and EMF renderers.
• Adds undoable inner-axis simplification without overwriting unrelated or
  explicit axis overrides.
• Adds Frame and Visual spacing modes, with Visual as the default.
• Interprets Visual gutter values as requested minimum data-area clearance
  while keeping plot frames non-overlapping.
• Adds Tight, Normal, and Spacious minimum-spacing presets.
• Uses shared object-aware layout logic for Apply grid, two-way split, and
  tile retile operations.
• Exposes the new layout actions through the command catalog, Arrange menus,
  Ribbon, and Canvas settings.
• Safely rejects Simplify Inner Axes for non-grid free layouts.
• Updates the English and Simplified Chinese user manuals.

Tight requests a minimum of 2 mm but does not guarantee an exact 2 mm result:
axis furniture and physically constrained pages may require larger spacing.

Validation

• cargo pr-check
• npm run build in docs/

UI display logic

• New or changed actions go through the command catalog (CommandId +
  description) and are searchable in the command palette.
• Hiding happens only at Ribbon-group level; transient states use command
  availability and explanatory feedback.
• No layout changes are triggered by background events.
• Invalid free-layout axis simplification leaves state and undo history
  unchanged and explains how to proceed.
• Docs were updated in English and zh-CN for user-visible behavior.