refactor(styles): split theme.css into per-theme files (#656)

theme.css (16138 LOC) → 122 per-section files in src/styles/themes/ +
an index.css that imports them in original cascade order.

Concatenating all files via index.css reproduces the original byte stream
(+1 trailing newline, cosmetic).

Each top-level section header in theme.css (matching /^\/\* ─{3,}/)
becomes its own file, slugged from the header text (or the [data-theme]
selector found in the body when the header was a banner). Pure-separator
headers fold into the previous section so they don't create empty files.

Generated via /tmp/split-theme-css.mjs.

src/styles/themes/animations.css

@@ -0,0 +1,59 @@
+/* ─── Animations ─── */
+@keyframes fadeIn {
+  from {
+    opacity: 0;
+    transform: translateY(8px);
+  }
+
+  to {
+    opacity: 1;
+    transform: none;
+  }
+}
+
+@keyframes slideIn {
+  from {
+    opacity: 0;
+    transform: translateX(-12px);
+  }
+
+  to {
+    opacity: 1;
+    transform: translateX(0);
+  }
+}
+
+@keyframes pulse {
+
+  0%,
+  100% {
+    opacity: 1;
+  }
+
+  50% {
+    opacity: 0.5;
+  }
+}
+
+@keyframes spin {
+  to {
+    transform: rotate(360deg);
+  }
+}
+
+.animate-fade-in {
+  animation: fadeIn var(--transition-slow) both;
+}
+
+.animate-slide-in {
+  animation: slideIn var(--transition-slow) both;
+}
+
+.animate-pulse {
+  animation: pulse 1.5s ease infinite;
+}
+
+.animate-spin {
+  animation: spin 1s linear infinite;
+}
+