fix(dom): skip wide glyph continuation cells

[EfeDurmaz16]

Summary

• expose optional cell display width through the shared CellData interface
• preserve Ghostty width metadata for viewport and scrollback cells
• skip width-0 continuation cells in the DOM renderer without shifting cursor placement

Closes #71.

Verification

• pnpm --filter @wterm/core build
• pnpm --filter @wterm/dom test
• pnpm --filter @wterm/dom type-check
• pnpm --filter @wterm/ghostty type-check
• pnpm --filter @wterm/core type-check
• pnpm --filter @wterm/core test
• pnpm format:check

[vercel]

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