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/utility-classes.css

@@ -0,0 +1,29 @@
+/* ─── Utility Classes ─── */
+.glass {
+  background: var(--bg-glass, rgba(30, 30, 46, 0.75));
+  backdrop-filter: blur(20px);
+  -webkit-backdrop-filter: blur(20px);
+  border: 1px solid var(--border-subtle);
+}
+
+.gradient-text {
+  background: linear-gradient(135deg, var(--ctp-mauve), var(--ctp-blue));
+  -webkit-background-clip: text;
+  -webkit-text-fill-color: transparent;
+  background-clip: text;
+}
+
+.truncate {
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.visually-hidden {
+  position: absolute;
+  width: 1px;
+  height: 1px;
+  overflow: hidden;
+  clip: rect(0, 0, 0, 0);
+}
+