refactor(styles): split layout.css into per-section files (#658)

layout.css (3209 LOC) → 29 per-section files in src/styles/layout/ +
an index.css that imports them in original cascade order.

Same mechanic as theme.css + components.css splits: top-level sections
detected by single-dash or 3+ dash header decoration. Concatenating in
@import order reproduces the original byte stream (+1 trailing newline,
cosmetic).

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

src/styles/layout/transport-controls.css

@@ -0,0 +1,50 @@
+/* ─── Transport Controls ─── */
+.mp-controls {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  gap: var(--space-4);
+  padding: 8px 0 4px;
+  flex-shrink: 0;
+}
+
+.mp-ctrl-btn {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 52px;
+  height: 52px;
+  border-radius: 50%;
+  background: none;
+  border: none;
+  color: var(--text-secondary);
+  cursor: pointer;
+  transition: color var(--transition-fast), transform var(--transition-fast);
+}
+
+.mp-ctrl-btn:hover {
+  color: var(--text-primary);
+  transform: scale(1.08);
+}
+
+.mp-ctrl-sm {
+  width: 44px;
+  height: 44px;
+  color: var(--text-muted);
+}
+
+.mp-ctrl-play {
+  width: 64px;
+  height: 64px;
+  background: var(--accent);
+  color: var(--ctp-crust);
+  box-shadow: 0 4px 20px rgba(0, 0, 0, 0.4);
+}
+
+.mp-ctrl-play:hover {
+  color: var(--ctp-crust);
+  filter: brightness(1.1);
+  transform: scale(1.06);
+  box-shadow: 0 6px 28px rgba(0, 0, 0, 0.4), var(--shadow-glow);
+}
+