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/header.css

@@ -0,0 +1,36 @@
+/* ─── Header ─── */
+.mp-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  height: 52px;
+  flex-shrink: 0;
+}
+
+.mp-header-title {
+  font-size: 13px;
+  font-weight: 600;
+  letter-spacing: 0.04em;
+  text-transform: uppercase;
+  color: var(--text-muted);
+}
+
+.mp-back {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 44px;
+  height: 44px;
+  border-radius: 50%;
+  background: none;
+  border: none;
+  color: var(--text-secondary);
+  cursor: pointer;
+  transition: color var(--transition-fast), background var(--transition-fast);
+}
+
+.mp-back:hover {
+  background: var(--bg-hover);
+  color: var(--text-primary);
+}
+