fix(canvas): prevent unnecessary page scrolling

[mohamad-fallah]

Problem

The widget canvas remained vertically scrollable even when all widgets fit inside the viewport.

Root cause

The canvas always reserved two extra grid rows:

const totalGridRows = Math.max(6, maxWidgetRow + 2)

These empty rows increased the canvas height during normal use and created unnecessary vertical scrolling.

Changes

The canvas height now follows the last occupied widget row in normal mode.

Two extra rows are reserved only in edit mode so users can still move widgets into empty space.

Layouts containing widgets below the viewport continue to scroll normally.

Left out

No widget positions, collision behavior, responsive layout rules, or visual styles were changed.

Testing

• npm run compile passed
• npm run build passed
• The changed file passed the project formatter check
• npm test could not start because bun is unavailable on PATH

Visual checklist

• A layout that fully fits inside the viewport should not scroll.
• A layout with a widget extending below the viewport should scroll.
• Edit mode should still show two extra rows for moving widgets.

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