feat: Add Manage page, light/dark theme toggle, and URL sharing

- Move Save, Load, Export, Import buttons to a dedicated Manage page
- Add light theme with warm parchment palette; auto-detect via prefers-color-scheme, persisted to localStorage
- Add URL sharing: sheet state is deflate-raw compressed, base64-encoded, and stored in ?c= parameter; auto-save is suppressed when viewing a shared URL
- Extract JS to fabula-ultima-sheet.js

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

fabula-ultima-sheet.css

@@ -964,6 +964,55 @@ input[type="number"] {
   font-weight: 600;
 }
 
+/* ── URL BANNER ──────────────────────────────────── */
+.url-banner {
+  background: var(--surface2);
+  border-bottom: 1px solid var(--teal-dim);
+  padding: 10px 32px;
+  font-family: var(--font-body);
+  font-size: 0.9rem;
+  color: var(--text-dim);
+  display: flex;
+  align-items: center;
+  gap: 10px;
+}
+
+.url-banner strong {
+  color: var(--text);
+}
+
+.url-banner-icon {
+  color: var(--teal);
+  font-size: 0.85rem;
+  flex-shrink: 0;
+}
+
+/* ── MANAGE PAGE ─────────────────────────────────── */
+.manage-grid {
+  display: grid;
+  grid-template-columns: 1fr 1fr;
+  gap: 20px;
+  max-width: 800px;
+}
+
+.manage-desc {
+  font-size: 0.9rem;
+  color: var(--text-dim);
+  line-height: 1.6;
+  margin-bottom: 18px;
+}
+
+.manage-btn-row {
+  display: flex;
+  gap: 10px;
+  flex-wrap: wrap;
+}
+
+.btn-lg {
+  font-size: 0.6rem;
+  padding: 11px 22px;
+}
+
 /* ── LIGHT THEME ─────────────────────────────────── */
 [data-theme="light"] {
   --bg: #f4efe4;