Ask for an optional contact email, with separate update and follow-up consent

Problem reports arrive with no way to reply. People can now leave an email
address with two separate opt-ins: occasional update notices, and follow-up
questions from the maintainer.

- ui/frame_contact.py keeps the address and choices locally and sends each
  change privately to PostHog as a contact_consent event under its own random
  contact id; removing the address sends a withdrawal without it. Changes made
  offline wait and are retried.
- A one-time, dismissible prompt appears after the Frame first connects; No
  thanks and showing it once are both remembered.
- Privacy & updates gains a Contact email section to add, change or remove it.
- The report form's contact field now goes with a report only when "may
  contact me with follow-up questions" is ticked (contact_followup).
- frame_report.py contacts [updates|followup] lists who agreed to what,
  using the newest event per copy.
- docs/privacy.md says what is collected, why, where and how to remove it.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

ui/server.py

@@ -50,6 +50,7 @@ import frame_catalog  # noqa: E402
 import frame_devices  # noqa: E402
 import frame_steamgriddb
 import frame_comfort  # noqa: E402
+import frame_contact  # noqa: E402
 import frame_host  # noqa: E402
 import frame_link  # noqa: E402
 import frame_macview  # noqa: E402
@@ -2151,6 +2152,7 @@ POST = {
         "/api/webinstall/check": webinstall_check, "/api/webinstall/start": webinstall_start,
         "/api/webinstall/cancel": webinstall_cancel,
         "/api/telemetry": frame_telemetry.update_settings, "/api/telemetry/event": frame_telemetry.page_event,
+        "/api/contact": frame_contact.save, "/api/contact/prompt": frame_contact.prompt,
         "/api/report/preview": report_preview, "/api/report": report_send, "/api/macview": macview_action, "/api/panels": panels_action,
         "/api/devices": lambda body: devices_post(body)}
 
@@ -2380,6 +2382,8 @@ class Handler(BaseHTTPRequestHandler):
                 self.send_json(macview_state(parse_qs(url.query)))
             elif path == "/api/telemetry":
                 self.send_json(frame_telemetry.state())
+            elif path == "/api/contact":
+                self.send_json(frame_contact.state())
             elif path == "/api/computer/state":
                 self.send_json(json.loads(ssh("python3 -", stdin=(HERE / "frame_computer.py").read_text(), timeout=20)))
             elif path == "/api/status":
@@ -2644,6 +2648,7 @@ def main():
     sweep_tmp()
     threading.Thread(target=apk_search.warm, daemon=True).start()  # big indexes download before the first search
     frame_telemetry.start()
+    frame_contact.start()
     global LINK, _ONE_SERVER
     if not LOCAL:
         if not PRIVATE:  # a private server only uses the headsets (see one_server)