feat: customisation studio layout

[lyon-1050]

What was implemented

1. CustomizationStudio.test.tsx — duplicate text match

When saveState is 'saving', the text "Saving…" appeared in two
places: the status and the Save . screen.getByText
throws when it finds multiple matches. Fixed by switching to
getAllByText(...).length > 0.

2. useCustomizationStudio.test.ts — waitFor timeouts with fake
   timers

vi.useFakeTimers() was applied globally in beforeEach, which froze
the microtask queue and caused waitFor (which polls via setTimeout
internally) to hang indefinitely. Fixed by removing the global
fake timer setup and only enabling fake timers (with {
shouldAdvanceTime: true }) in the single test that actually needs
them — the auto-save debounce test — then restoring real timers
immediately after.

closes #16

[drips-wave]

@lyon-1050 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits