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/mobile/app.json

@@ -16,6 +16,9 @@
     "ios": {
       "supportsTablet": false,
       "bundleIdentifier": "com.sauci.app",
+      "associatedDomains": [
+        "applinks:sauci.app"
+      ],
       "config": {
         "usesNonExemptEncryption": false
       },
@@ -194,6 +197,23 @@
         "android.permission.USE_BIOMETRIC",
         "android.permission.USE_FINGERPRINT",
         "com.android.vending.BILLING"
+      ],
+      "intentFilters": [
+        {
+          "action": "VIEW",
+          "autoVerify": true,
+          "data": [
+            {
+              "scheme": "https",
+              "host": "sauci.app",
+              "pathPrefix": "/join"
+            }
+          ],
+          "category": [
+            "BROWSABLE",
+            "DEFAULT"
+          ]
+        }
       ]
     },
     "web": {