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/components/lanes/LaneDiffView.tsx

@@ -45,7 +45,7 @@ function Code({ text, lang, emph }: { text: string; lang: string; emph?: [number
     return (
       <>
         <Code text={text.slice(0, from)} lang={lang} />
-        <span className="rounded-sm bg-white/15">
+        <span className="rounded-sm bg-fg-primary/15">
           <Code text={text.slice(from, to)} lang={lang} />
         </span>
         <Code text={text.slice(to)} lang={lang} />
@@ -120,12 +120,21 @@ function Row({ tint, children }: { tint: string; children: React.ReactNode }) {
   return <div className={`flex w-max min-w-full ${tint}`}>{children}</div>;
 }
 
+/** A /20 fill plus a 3px status bar at the row's left edge, so +/- rows still
+ *  separate from context at a glance on every theme (the /10 fill alone did not). */
 function rowTint(type: LaneDiffLine["type"] | null) {
-  if (type === "add") return "bg-status-success/10";
-  if (type === "del") return "bg-status-danger/10";
+  if (type === "add")
+    return "bg-status-success/20 shadow-[inset_3px_0_0_rgb(var(--status-success))]";
+  if (type === "del") return "bg-status-danger/20 shadow-[inset_3px_0_0_rgb(var(--status-danger))]";
   return "";
 }
 
+function markerClass(type: LaneDiffLine["type"]) {
+  if (type === "add") return "font-bold text-status-success";
+  if (type === "del") return "font-bold text-status-danger";
+  return "text-fg-muted";
+}
+
 function HunkHeader({ hunk }: { hunk: LaneDiffHunk }) {
   const at = hunk.header.split("@@")[1];
   return (
@@ -153,7 +162,7 @@ function UnifiedHunk({ hunk, lang }: { hunk: LaneDiffHunk; lang: string }) {
           <Gutter n={line.oldLine} />
           <Gutter n={line.newLine} />
           <span className="whitespace-pre px-2">
-            <span className="select-none text-fg-muted">
+            <span className={`select-none ${markerClass(line.type)}`}>
               {line.type === "add" ? "+" : line.type === "del" ? "-" : " "}
             </span>
             <Code text={line.text} lang={lang} emph={emph.get(i)} />