fix(account): keep section changes in /dashboard and /settings from looking like a reload

[SomethingNew71]

Summary

/settings and /dashboard are nested routes: the hero and the sidebar stay mounted, and only the section content swaps. Two site-wide defaults still made a section change look like a full reload:

• experimental.viewTransition cross-faded the whole document on every navigation.
• Nuxt's default scroll behaviour jumped back to the hero.

This PR:

• adds app/plugins/account-shell-navigation.client.ts, which sets viewTransition: false and scrollToTop: false on to.meta in beforeEach, for a section change inside one shell only (isAccountShellSectionChange). Other navigations keep the fade.
• makes <AccountShell> bring the new section's start into view only when the visitor had scrolled past it. scroll-mt-24 clears the sticky header.
• records in the rules and invariants why definePageMeta does not work here: the macro has no auto-imports, so the helper was undefined and Nuxt hard-reloaded.

Test plan

• bun run test: 6426 pass. New tests cover the helper, the plugin and the shell scroll.
• Typecheck matches the baseline. The format check and the shadowed auto-import sweep are clean.
• Dev server, signed out:
  • A section change runs no view transition (0 startViewTransition calls), keeps the scroll position and keeps the same hero element.
  • Scrolled past the section start, a section change lands on the section start.
  • Leaving to /about still fades.
• Signed in: switch between the settings and dashboard sections in a real browser.

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