feat(ui): Appearance settings — dark/light/system mode and named themes; fix Add lane silently doing nothing (#42)

- Settings → Appearance: mode Dark / Light / System (follows the OS live) and
  a named theme remembered per mode — dark: Default, Darcula, Apple Dark;
  light: Default, Apple Light, IntelliJ Light, Canvas. Each theme is a
  CSS-variable override block keyed by `data-theme` on <html>; the sidebar
  quick toggle gains System and swaps between the two remembered themes.
- An inline boot script in index.html applies the stored mode and theme
  before first paint, so a reload no longer flashes the default colours.
- Diff +/- rows are easier to tell apart on every theme: a /20 status tint,
  a 3px status bar and a bold coloured marker; the changed-span emphasis no
  longer vanishes on light themes.
- #42: the Add-lane modal (Repo mode / "Current branch") asks
  POST /api/lanes/ensure with `exact: true`. Without it a folder nested inside
  an existing lane resolved to that parent lane with created:false, so the
  modal closed and no new card appeared. When the folder really already has a
  lane, the modal now says which one instead of closing.

Closes #42

client/src/lib/highlight.ts

@@ -1037,7 +1037,7 @@ export function highlight(source: string, lang: string): Token[] {
  *   (`--syn-*` in index.css), not on fixed palette steps: the dark set is the
  *   pastel one this file has always used, and light mode takes the same hues
  *   four steps darker, because a pastel keyword on a light code block is
- *   unreadable. Diff rows keep their faint ±10% tint, which works on both.
+ *   unreadable. Diff rows take a /20 status tint, pre-mixed per theme.
  */
 export function tokenClass(type: TokenType): string {
   switch (type) {
@@ -1068,9 +1068,9 @@ export function tokenClass(type: TokenType): string {
     case "boolean":
       return "text-syntax-number"; // literals grouped with numbers by colour
     case "diff-add":
-      return "text-syntax-add bg-emerald-500/10"; // green text + faint green row tint
+      return "text-syntax-add bg-status-success/20"; // green text + theme-aware green row tint
     case "diff-del":
-      return "text-syntax-del bg-red-500/10"; // red text + faint red row tint
+      return "text-syntax-del bg-status-danger/20"; // red text + theme-aware red row tint
     case "diff-meta":
       return "text-syntax-keyword";
     case "plain":