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/Sidebar.tsx

@@ -71,6 +71,7 @@ import {
   ChevronUp,
   ChevronDown,
   Sun,
+  Monitor,
   Moon,
 } from "lucide-react";
 import type { LucideIcon } from "lucide-react";
@@ -177,7 +178,7 @@ interface SidebarProps {
 
 export function Sidebar({ wsConnected, collapsed, onToggle }: SidebarProps) {
   const { t, i18n } = useTranslation();
-  const { theme, setTheme, toggleTheme } = useTheme();
+  const { theme, mode, setMode, toggleTheme } = useTheme();
   // Track whether nav items are clipped by overflow so we can render
   // chevron affordances pointing toward the hidden items. Recomputed on
   // scroll, resize, and any structural change (e.g. collapse toggle).
@@ -545,13 +546,13 @@ export function Sidebar({ wsConnected, collapsed, onToggle }: SidebarProps) {
                 <p className="px-1 text-[10px] font-semibold uppercase tracking-wider text-fg-muted">
                   {t("nav:theme")}
                 </p>
-                <div className="mt-2 grid grid-cols-2 gap-1">
-                  {(["dark", "light"] as const).map((option) => {
-                    const active = option === theme;
+                <div className="mt-2 grid grid-cols-3 gap-1">
+                  {(["dark", "light", "system"] as const).map((option) => {
+                    const active = option === mode;
                     return (
                       <button
                         key={option}
-                        onClick={() => setTheme(option)}
+                        onClick={() => setMode(option)}
                         aria-pressed={active}
                         aria-label={t(`nav:themeNames.${option}`)}
                         title={t(`nav:themeNames.${option}`)}
@@ -563,8 +564,10 @@ export function Sidebar({ wsConnected, collapsed, onToggle }: SidebarProps) {
                       >
                         {option === "dark" ? (
                           <Moon className="w-3.5 h-3.5" />
-                        ) : (
+                        ) : option === "light" ? (
                           <Sun className="w-3.5 h-3.5" />
+                        ) : (
+                          <Monitor className="w-3.5 h-3.5" />
                         )}
                       </button>
                     );