Fix dashboard lint failures blocking clean CI

[semyonfox]

Why

The dashboard currently builds, but pnpm lint fails. This means a future CI gate would be red immediately and the current React hook issues can hide real runtime bugs.

Evidence

Local pnpm lint failures:

• dashboard/src/auth/AuthProvider.tsx:14: react-refresh/only-export-components because the context is exported from the component module.
• dashboard/src/auth/AuthProvider.tsx:41: react-hooks/set-state-in-effect around refreshUser().finally(() => setLoading(false)).
• dashboard/src/billing/AccountPage.tsx:24: react-hooks/rules-of-hooks because useEffect is called after if (!user) return null.

There is also a React Compiler warning around TanStack Table in dashboard/src/components/StationTable.tsx:66.

Proposed work

• Move AuthContext into a small separate module or otherwise satisfy Fast Refresh.
• Restructure AuthProvider initialization so the effect does not synchronously cascade state updates in the flagged pattern.
• Move the AccountPage early return below hooks, or split authenticated content into a child component.
• Decide whether to suppress/document the TanStack Table compiler warning or adjust the lint config for known incompatible-library cases.

Acceptance criteria

• pnpm lint exits 0.
• pnpm build still succeeds.
• Account page usage loading still works for authenticated users and renders nothing/redirects correctly for anonymous users.