Add calculator UX enhancements and submission tracking

- Button loading spinner + disable on submit to prevent duplicate AI calls
- Email field and opt-in consent checkbox on calculator form (checked by default)
- Privacy assurance blurb below opt-in
- Refinement blurb on results screen explaining live cash-flow optimization
- calc_submissions SQLite table stores form inputs, computed results, AI text, email, opt-in
- GET /api/calc-submissions endpoint (x-admin-key protected) to retrieve submissions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

styles.css

@@ -840,6 +840,50 @@ body {
 .calc-recalc { font-size: 14px; color: var(--gray-500); }
 .calc-recalc:hover { color: var(--gray-200); }
 
+/* ── Calc email + opt-in ── */
+.calc-email-row { margin-top: 20px; display: flex; flex-direction: column; gap: 10px; }
+.calc-field--full { grid-column: 1 / -1; }
+.calc-optional { font-weight: 400; color: var(--gray-600); font-size: 11px; }
+.calc-optin-label {
+  display: flex; align-items: flex-start; gap: 10px;
+  font-size: 13px; color: var(--gray-300); cursor: pointer; line-height: 1.5;
+}
+.calc-optin-label input[type="checkbox"] {
+  margin-top: 2px; accent-color: var(--blue); flex-shrink: 0;
+  width: 15px; height: 15px; cursor: pointer;
+}
+.calc-privacy {
+  font-size: 11px; color: var(--gray-600); line-height: 1.5;
+  border-left: 2px solid rgba(255,255,255,0.06); padding-left: 10px; margin: 0;
+}
+
+/* ── Calc button spinner ── */
+.calc-btn-loading { display: flex; align-items: center; gap: 8px; }
+.calc-spinner {
+  display: inline-block; width: 14px; height: 14px;
+  border: 2px solid rgba(255,255,255,0.3);
+  border-top-color: #fff;
+  border-radius: 50%;
+  animation: spin 0.7s linear infinite;
+}
+@keyframes spin { to { transform: rotate(360deg); } }
+
+/* ── Results refinement blurb ── */
+.calc-refine-blurb {
+  display: flex; gap: 12px; align-items: flex-start;
+  background: rgba(14,165,233,0.07);
+  border: 1px solid rgba(14,165,233,0.18);
+  border-radius: 10px;
+  padding: 14px 16px;
+  margin-bottom: 20px;
+}
+.calc-refine-icon {
+  font-size: 18px; color: var(--sky); flex-shrink: 0; margin-top: 1px;
+}
+.calc-refine-blurb p {
+  font-size: 12.5px; color: var(--gray-400); line-height: 1.6; margin: 0;
+}
+
 /* ---- Screenshot Carousel ---- */
 .screenshot-carousel {
   width: 100%;