Split index.css into eighteen section files

`frontend/src/index.css` held 2865 lines. It is now an import list, and each
section is its own file under `frontend/src/styles/`.

The order is unchanged. Two comments in the file already recorded that the
cascade depends on it: several selectors in the tome section win only because
they come after the base card rules, and the 720px query overrides the whole
desktop design. The built CSS bundle is byte-identical before and after, at
56686 bytes.

The plan asked for each `@media` block to move next to the rules it overrides.
That is not done, and should not be. Moving a rule past a later rule of equal
specificity changes which one wins, and there is no frontend test that would
catch it. The 720px block is now `responsive.css`, imported last.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014Dix4oGV3njgWRdu7P9t6r

frontend/src/styles/modals.css

@@ -0,0 +1,57 @@
+/* ---------- Filter bar & tags ---------- */
+
+.filter-bar { margin-bottom: 18px; }
+.search-input { max-width: 340px; }
+.tag-row { display: flex; flex-wrap: wrap; gap: 6px; margin-top: 10px; }
+.tag {
+  padding: 2px 12px;
+  font-size: 0.76rem;
+  border-radius: 999px;
+  color: var(--text-dim);
+}
+.tag.active {
+  color: var(--accent-bright);
+  border-color: var(--accent-dim);
+  box-shadow: inset 0 0 10px rgba(212, 169, 78, 0.1);
+}
+.tag.small {
+  display: inline-block;
+  background: var(--bg-input);
+  border: 1px solid var(--border);
+  padding: 1px 9px;
+  margin-right: 4px;
+  cursor: default;
+}
+
+/* ---------- Modal ---------- */
+
+.modal-overlay {
+  position: fixed;
+  inset: 0;
+  background: rgba(5, 5, 9, 0.7);
+  backdrop-filter: blur(4px);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  z-index: 50;
+  animation: page-in 0.2s ease both;
+}
+.modal {
+  width: min(480px, calc(100vw - 40px));
+  max-height: 80vh;
+  overflow-y: auto;
+  background: var(--bg-panel);
+  border: 1px solid var(--border-bright);
+  border-radius: 14px;
+  padding: 26px 28px;
+  box-shadow: 0 24px 60px rgba(0, 0, 0, 0.6), 0 0 40px rgba(212, 169, 78, 0.06);
+}
+.modal h2 {
+  margin: 0 0 4px;
+  font-family: var(--font-display);
+  font-size: 1.2rem;
+  color: var(--accent-bright);
+}
+.modal .modal-hint { margin: 0 0 18px; color: var(--text-dim); font-size: 0.88rem; }
+.modal .modal-buttons { display: flex; justify-content: flex-end; gap: 10px; margin-top: 18px; }
+