Extract magic number to named constant with documentation

[Copilot]

Addresses code review feedback to replace the hardcoded 0.92 fill ratio threshold with a named constant.

Changes

• Added MATRIX_FILL_RATIO_THRESHOLD constant at module level with documentation explaining the 92% threshold determines when to left-align matrix layout (sidebar visible) vs center it
• Updated fill ratio comparison to use the constant instead of the magic number

// Before
const shouldAlignLeft = sidebarCollapsed
  ? (containerWidth > availableWidth)
  : ((containerWidth > availableWidth) || (fillRatio < 0.92));

// After
const MATRIX_FILL_RATIO_THRESHOLD = 0.92;
// ...
const shouldAlignLeft = sidebarCollapsed
  ? (containerWidth > availableWidth)
  : ((containerWidth > availableWidth) || (fillRatio < MATRIX_FILL_RATIO_THRESHOLD));

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