feat(frontend): capture marketing-site triggerprompt into sessionStorage (#13248)

### Why / What / How

**Why.** The marketing site hero CTA hands a user-entered prompt to the
platform so the chat starts with it pre-submitted. The previous handoff
used `https://platform.agpt.co/copilot?autosubmit=true#prompt=...`. URL
hashes are never sent to the server, so when the Supabase middleware
redirects an unauthenticated visitor from `/copilot` to
`/login?next=/copilot?autosubmit=true`, the `#prompt=` fragment is
dropped. New users always lost their prompt before reaching the chat.

**What.** Switch the handoff to a `?triggerprompt=` query param read on
`/signup` and `/login` mount. A small `useCaptureMarketingPrompt` hook
stashes the value into sessionStorage and strips it from the URL. The
existing `useWorkflowImportAutoSubmit` on `/copilot` already consumes
that sessionStorage key, so the prompt survives the auth → onboarding →
`/copilot` redirect chain and auto-submits once the user lands in the
chat.

**How.**
- `useCaptureMarketingPrompt` (new): mount-once effect that reads
`?triggerprompt=` from the URL, writes `importWorkflowPrompt` +
`importWorkflowAutosubmit` to sessionStorage, and uses
`history.replaceState` to remove the param so subsequent navigations are
clean.
- Wired into `useSignupPage` and `useLoginPage` as the first hook call,
so capture happens before any logged-in short-circuit, password-form
submit, or OAuth redirect (sessionStorage survives the OAuth round-trip
since the user returns to the same origin).
- `useWorkflowImportAutoSubmit` now also reads the autosubmit flag from
sessionStorage. URL `?autosubmit=true` continues to win when explicitly
set (workflow import path), but the marketing handoff needs the
storage-side flag because onboarding wipes URL params before we reach
`/copilot`.

Marketing-site change (`/signup?triggerprompt=...` instead of
`/copilot?autosubmit=true#prompt=...`) shipped separately in the
marketing repo.

### Changes 🏗️

- New `src/hooks/useCaptureMarketingPrompt.ts`
- `useSignupPage` and `useLoginPage` call the new hook on mount
- `useWorkflowImportAutoSubmit` consumes the `importWorkflowAutosubmit`
sessionStorage flag in addition to the URL param

### 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:
- [ ] New unauthenticated user: open
`https://platform.agpt.co/signup?triggerprompt=Hello%20world`, complete
email + password signup, finish onboarding, confirm `/copilot`
auto-submits "Hello world" as the first message.
  - [ ] Same flow via Google OAuth.
- [ ] Existing logged-in user: open
`https://platform.agpt.co/signup?triggerprompt=Hello`, confirm
OnboardingProvider routes through to `/copilot` and the prompt
auto-submits.
- [ ] Existing logged-in user with onboarding incomplete: same URL,
confirm onboarding completes then `/copilot` auto-submits.
- [ ] Refreshing `/signup?triggerprompt=...` after the URL is stripped
does not lose the prompt (sessionStorage persists until consumed on
`/copilot`).
- [ ] Existing workflow-import flow
(`/copilot?source=import&autosubmit=true` + sessionStorage) still
auto-submits the imported workflow prompt.
- [ ] Existing `/copilot#prompt=...&autosubmit=true` deep link (e.g.
SitrepItem, LibraryAgentCard) still works.

#### For configuration changes:

- [x] `.env.default` is updated or already compatible with my changes
- [x] `docker-compose.yml` is updated or already compatible with my
changes
- [x] I have included a list of my configuration changes in the PR
description (under **Changes**)

_No configuration changes._