fix(editor): make canvas mouse scroll use shift-wheel sideways

[DavidBabinec]

What changed

• Made the design canvas mouse scroll model explicit: wheel scrolls vertically, Shift+wheel scrolls sideways, and Ctrl/Cmd+wheel still zooms.
• Removed middle-button dragging as a canvas pan gesture, including the iframe pointer bridge path.
• Added a shared canvas input policy helper and focused regression coverage for Shift+wheel and middle-button drag behavior.
• Updated canvas iframe and DnD docs to stop documenting middle-click canvas panning.

Why

The canvas still treated middle-button dragging as a first-class mouse pan shortcut, which is not the expected desktop mouse scrolling model. Sideways mouse scrolling should be keyboard-modified wheel input instead.

Impact

Desktop mouse users can pan sideways with Shift+wheel across the site canvas. Space-left-drag remains available as the intentional keyboard-modified drag pan gesture; middle-button dragging no longer pans the canvas.

Verification

• bun test src/__tests__/canvas/useCanvasWheelSync.test.tsx passed: 5 tests, 0 failures.
• bun run build passed.
• bun run lint passed.
• bunx react-doctor@latest --verbose --diff passed with no changed-file issues.
• bunx react-doctor@latest --verbose --scope changed --base main passed with no changed-file issues.
• bun test was run and still has the pre-existing unrelated failure also present on the fresh baseline: OnboardingPanel framework import > dispatches a CMS site reload after a successful import so the editor refetches (expects a button named /import framework/i, rendered button is Import).