[scheduler] Define responsive strategy and implement the base

[rita-codes]

First step of the responsive scheduler: review the design once it's ready, decide the approach, and lay the foundation the responsive views will build on top of.

Steps:

1. Review the final design .
2. Decide the strategy, two main precedents in MUI X:
   • Pickers pattern: separate <DesktopScheduler> / <MobileScheduler> selected via desktopModeMediaQuery prop (same public API)
   • Data Grid pattern: single tree that adapts internally via breakpoint constants + media queries
3. Implement the base: breakpoint strategy, shared layout/toolbar/navigation adjustments — whatever the chosen approach requires before specific views plug in

Out of scope:

• Specific responsive view implementations (tracked in [scheduler] Add a week/day responsive view #21829, [scheduler][stretch] Mobile month + agenda view #21584)

Blocks: #21829, #21584

Note: The design is still TBD.