feat(invite-funnel): add web join page and mobile deep-link hand-off

Adds a shareable https://sauci.app/join/{code} link as an alternative to
manually typing an 8-character invite code:

- apps/web: /join/[code] page that validates the code shape client-side,
  attempts the app.sauci:// scheme link, and falls back to a large code
  display with copy button plus App Store/Play Store links; PostHog events
  for page view, code copy, and store-button clicks.
- apps/web: apple-app-site-association and assetlinks.json under
  /.well-known so iOS Universal Links and Android App Links can verify
  sauci.app for /join/*. The Android release cert SHA-256 fingerprint is a
  placeholder pending the real value.
- apps/mobile: associated domains (iOS) and an intent filter (Android) for
  https://sauci.app/join/*, alongside the existing app.sauci scheme.
- apps/mobile: parse incoming join links, pre-fill and highlight the code on
  the pairing screen, stash the code for signed-out/mid-onboarding users and
  apply it once they reach pairing, and offer a once-per-session clipboard
  hand-off that is never auto-applied.
- apps/mobile: share message now leads with the join link, keeping the raw
  code visible as a fallback.
- Analytics: invite_link_opened, pairing_code_prefilled (mobile);
  join_page_viewed, join_page_code_copied, join_page_store_button_clicked
  (web).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/web/lib/appLinks.ts

@@ -0,0 +1,15 @@
+// App store and deep-link destinations used by the invite join page
+// (apps/web/app/join/[code]/page.tsx). These are the real, live listing URLs
+// already used by components/Hero.tsx, components/CTA.tsx,
+// components/Navbar.tsx, and components/Pricing.tsx.
+
+export const APP_STORE_URL = 'https://apps.apple.com/gb/app/sauci-couple-games-romance/id6757159885'
+export const PLAY_STORE_URL = 'https://play.google.com/store/apps/details?id=com.sauci.app'
+
+export function buildInviteSchemeUrl(code: string): string {
+  return `app.sauci://join?code=${encodeURIComponent(code)}`
+}
+
+export function buildInviteUniversalUrl(code: string): string {
+  return `https://sauci.app/join/${encodeURIComponent(code)}`
+}

apps/web/lib/inviteCode.test.ts

@@ -0,0 +1,24 @@
+import { describe, expect, it } from 'vitest'
+import { isValidInviteCode, normalizeInviteCode } from './inviteCode'
+
+describe('isValidInviteCode', () => {
+  it('accepts 8-character alphanumeric codes', () => {
+    expect(isValidInviteCode('ABCD1234')).toBe(true)
+    expect(isValidInviteCode('abcd1234')).toBe(true)
+  })
+
+  it('rejects codes with the wrong length or characters', () => {
+    expect(isValidInviteCode('ABC123')).toBe(false)
+    expect(isValidInviteCode('ABCD12345')).toBe(false)
+    expect(isValidInviteCode('ABCD-123')).toBe(false)
+    expect(isValidInviteCode('')).toBe(false)
+    expect(isValidInviteCode(null)).toBe(false)
+    expect(isValidInviteCode(undefined)).toBe(false)
+  })
+})
+
+describe('normalizeInviteCode', () => {
+  it('trims and uppercases', () => {
+    expect(normalizeInviteCode('  abcd1234  ')).toBe('ABCD1234')
+  })
+})

apps/web/lib/inviteCode.ts

@@ -0,0 +1,15 @@
+// Shared invite code shape validation for the web join page. Mirrors the
+// mobile app's invite code shape (apps/mobile/src/lib/inviteLink.ts) and the
+// standalone API's invite_code column. This module only validates the shape
+// client-side; it never calls a private API.
+
+const INVITE_CODE_PATTERN = /^[A-Za-z0-9]{8}$/
+
+export function isValidInviteCode(code: string | null | undefined): boolean {
+  if (!code) return false
+  return INVITE_CODE_PATTERN.test(code.trim())
+}
+
+export function normalizeInviteCode(code: string): string {
+  return code.trim().toUpperCase()
+}