fix(ui): persist nav open state and honor preference at sidebar widths

[vikas-x7]

What?

The admin nav was force-closed at viewports ≤1440px (l breakpoint) even though that range renders the nav as a sidebar, not a modal. The stored nav preference was only read above 1440px, and toggling the nav never wrote the preference at all (the v3 NavToggler that handled this was removed in #17212).

Fixes #17531

Why?

On any project whose desktop breakpoint is 1440px, the sidebar was never open on first load, the user's saved preference was ignored, and "remember my choice" did not exist. The unconditional close effect used the l breakpoint, but the comment/behaviour is about modal widths only.

How?

• packages/ui/src/elements/Nav/context.tsx:
  • Read the nav preference on every sidebar width (not just >1440px). Gate changed from largeBreak === false to midBreak === false && smallBreak === false.
  • Only force-close the nav on the genuinely modal widths: midBreak === true || smallBreak === true. The l breakpoint (1440px) is no longer treated as a modal width.
• packages/ui/src/elements/AppHeader/index.tsx:
  • The sidebar toggle now persists the preference via setPreference(PREFERENCE_KEYS.NAV, { open }, true) (same pattern the v3 NavToggler used, merge: true preserves nav-group prefs), gated to sidebar widths only so programmatic/modal closes are not persisted.

Tests

• Added an e2e regression test in test/admin/e2e/general/e2e.spec.ts (nav — should be open by default at sidebar widths and persist the toggle preference) at 1280px: asserts nav is open by default, and that closing/reopening + reload persists.
• Existing openNav/closeNav e2e helpers already handle both default states, so the viewport change does not break the surrounding tests.
• ESLint: no new issues from these changes. Typecheck (tsc -p packages/ui/tsconfig.json --noEmit): 0 errors.