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/mouse-event-dnd-cursor-psy-dragging.css

@@ -0,0 +1,10 @@
+/* ─── Mouse-event DnD cursor (psy-dragging) ──────────────────────
+   Applied by DragDropProvider while a mouse-based drag is active.
+   Overrides cursor globally to "grabbing" and prevents text-
+   selection / stray pointer events on child elements. ─────────── */
+body.psy-dragging,
+body.psy-dragging * {
+  cursor: grabbing !important;
+  user-select: none !important;
+}
+