ui: collapsible settings sections + welcome-email + larger auth inputs

Settings page tidy-up driven by user feedback that it had grown too busy:

  - Each section (Import, Invite, Email digests, Cloud sync) is now a
    native <details>/<summary> accordion. Import stays open by default
    because /settings#import is the deep-link target from the dashboard
    CTA; the others collapse so the page lands quiet.
  - Manage subscription is a right-aligned gear-icon button instead of
    a rectangular text button — the descriptive copy moves into the
    tooltip. Frees up the Tier row of visual weight.

Auth + modal inputs were too small (verify code box, portfolio restore
PIN): the auth-card selector now covers text inputs as well, and a new
.modal-input class standardises 16px / 12px-padding fields used in the
cloud-sync enable modal and the portfolio restore prompt.

The verify page no longer carries the "Email me the digest" checkbox —
it was misleading on repeat logins (server-side it only applied on
first sign-up but rendered every time). Default-opt-in lives in the
User row at creation; per-user changes happen on /settings. First
successful verify now triggers a one-shot welcome email explaining the
digest cadence and pointing at /settings for opt-out; SMTP failure is
logged but does not block the login.

Tests rewritten to cover the new welcome-email path:
  - first login sends exactly one welcome email
  - returning user gets none
  - SMTP failure does not break the redirect
  - regression guard: returning user who opted out stays opted out

Also lands the paddle merchant-summary doc that was written earlier
during the Paddle → Polar → Stripe onboarding pivot.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/templates/verify.html

@@ -29,14 +29,7 @@
     <form method="post" action="/verify" autocomplete="off">
       <label>Verification code
         <input type="text" name="code" inputmode="numeric" pattern="[0-9]{6}"
-               minlength="6" maxlength="6" required autofocus
-               style="font-family:var(--font-mono); letter-spacing:0.4em; text-align:center;">
-      </label>
-      <label style="display:block; margin:14px 0 0; font-size:12.5px; color:var(--muted); line-height:1.55;">
-        <input type="checkbox" name="subscribe_to_digests" value="on" checked
-               style="vertical-align:middle; margin-right:6px;">
-        Email me the digest — daily for paid, Sunday for everyone.
-        One-click unsubscribe in every email.
+               minlength="6" maxlength="6" required autofocus>
       </label>
       <button type="submit">Verify</button>
     </form>