fix(frontend): drop /signup → / hop that flashes /copilot before /onboarding (#13154)

### Why / What / How

**Why**: New users who finish the signup form, and logged-in users who
land on `/signup` or `/login`, briefly see the `/copilot` homescreen
before being sent to `/onboarding`. The bounce is `/signup` → `/` →
`/copilot` → `/onboarding`, and each hop renders before the next
redirect kicks in — the intermediate `/copilot` render is the flash
users see. The LD angle is real: `LaunchDarklyProvider`
unmounts/remounts its subtree on every `isUserLoading` flip, which
resets `OnboardingProvider.hasInitialized` and delays the corrective
redirect behind another awaited `getV1CheckIfOnboardingIsCompleted`
call. While that promise is in flight, `/copilot` sits on screen.

**What**: Stop `useSignupPage` / `useLoginPage` from pushing to `"/"`
when the user is logged in (which always redirects to `/copilot` via
`app/page.tsx`). Let `OnboardingProvider` make the routing decision, and
extend it to also handle completed users sitting on `/signup` or
`/login` so they aren't stranded on the auth-page loader.

**How**:
1. Narrow the auth-page `useEffect`s so they only honour an explicit
`?next=` deep link. The generic "I'm logged in, get me off this page"
case is now routed by `OnboardingProvider`, which knows whether to send
the user to `/onboarding` or `/copilot`.
2. Add `isOnAuthRoute` to the `OnboardingProvider`'s "completed" branch
so a logged-in user with completed onboarding on `/signup` or `/login`
is sent straight to `/copilot` (otherwise they'd be stuck on
`LoadingSignup` forever now that the auth-page redirect is gone).

### Changes 🏗️

- `useSignupPage.ts`: skip the `router.push("/")` when no `?next=` is
set.
- `useLoginPage.ts`: same change for the `/login` mirror of the same
useEffect.
- `onboarding-provider.tsx`: introduce `isOnAuthRoute`; broaden the
completed-onboarding redirect to include `/signup` and `/login`; add it
to the effect dep list.

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [ ] I have tested my changes according to the test plan:
- [x] `pnpm format`, `pnpm lint`, `pnpm types`, `pnpm test:unit` all
pass on the branch (203 test files / 2660 tests passing)
- [ ] Fresh signup: confirm the user goes straight to `/onboarding`
welcome step with no `/copilot` flicker
- [ ] Re-visit `/signup` while logged in with incomplete onboarding:
should land on `/onboarding`, no `/copilot` flicker
- [ ] Re-visit `/signup` while logged in with completed onboarding:
should land on `/copilot`
- [ ] Re-visit `/signup?next=/library` while logged in: should land on
`/library`
  - [ ] Same matrix for `/login`

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>