Add in-app Settings page for secrets; move Gmail creds out of source

- settings.py: JSON-on-volume store with env fallback; secrets never rendered.
- /settings page (⚙️ in nav) to set Gmail address + app password + reminder prefs;
  password fields are write-only (blank keeps existing).
- reminders.py reads Gmail creds from Settings instead of a hardcoded password
  (resolves #2 in code — note the old password is still in git history; rotate it).
- mac_notification no longer crashes off-macOS; FLASK_DEBUG gates debug (#7).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/settings.html

@@ -0,0 +1,59 @@
+{% extends "base.html" %}
+{% block content %}
+<div style="max-width:640px;">
+  <h1>⚙️ Settings</h1>
+  <p style="color:#7A6E68;margin-bottom:1.25rem;">
+    Secrets are stored on the server (in the data volume) and are never shown
+    here once saved. Leave a password field blank to keep the current value.
+  </p>
+
+  {% if saved %}
+  <div class="alert-banner" style="background:#E7F3E9;border-color:#BFE0C5;margin-bottom:1rem;">
+    <span>✓ Settings saved.</span>
+  </div>
+  {% endif %}
+
+  <form method="POST" action="{{ url_for('settings_view') }}">
+    <fieldset style="border:1px solid #EDE8E2;border-radius:10px;padding:1rem 1.25rem;margin-bottom:1.25rem;">
+      <legend style="padding:0 .5rem;font-weight:600;">📧 Email reminders (Gmail)</legend>
+
+      <label style="display:block;margin-bottom:.9rem;">
+        <span style="display:block;font-size:.85rem;color:#7A6E68;margin-bottom:.25rem;">Gmail address</span>
+        <input type="email" name="gmail_address" value="{{ settings.gmail_address }}"
+               placeholder="you@gmail.com" style="width:100%;padding:.55rem;border:1px solid #cfcfd4;border-radius:8px;">
+      </label>
+
+      <label style="display:block;margin-bottom:.9rem;">
+        <span style="display:block;font-size:.85rem;color:#7A6E68;margin-bottom:.25rem;">
+          Gmail app password
+          {% if secrets_status.gmail_app_password %}
+            <strong style="color:#5A9E68;">· set ✓</strong>
+          {% else %}
+            <strong style="color:#C57A6E;">· not set</strong>
+          {% endif %}
+        </span>
+        <input type="password" name="gmail_app_password" autocomplete="new-password"
+               placeholder="{% if secrets_status.gmail_app_password %}•••••••••••• (leave blank to keep){% else %}16-character app password{% endif %}"
+               style="width:100%;padding:.55rem;border:1px solid #cfcfd4;border-radius:8px;">
+        <span style="display:block;font-size:.78rem;color:#B0A89E;margin-top:.25rem;">
+          Not your normal password — create an
+          <a href="https://myaccount.google.com/apppasswords" target="_blank" rel="noopener">app password</a> in your Google account.
+        </span>
+      </label>
+
+      <label style="display:block;margin-bottom:.9rem;">
+        <span style="display:block;font-size:.85rem;color:#7A6E68;margin-bottom:.25rem;">Send reminders to (optional)</span>
+        <input type="email" name="reminder_recipient" value="{{ settings.reminder_recipient }}"
+               placeholder="defaults to the Gmail address above" style="width:100%;padding:.55rem;border:1px solid #cfcfd4;border-radius:8px;">
+      </label>
+
+      <label style="display:flex;align-items:center;gap:.5rem;">
+        <input type="checkbox" name="reminders_enabled" {% if settings.reminders_enabled %}checked{% endif %}>
+        <span>Email reminders enabled</span>
+      </label>
+    </fieldset>
+
+    <button type="submit" class="btn btn-primary">Save settings</button>
+  </form>
+</div>
+{% endblock %}