Retone dark theme to warm gray #1a1a19 with layered surfaces

The dark palette was shadcn's cool zinc (#202023, hue ~286). Anchor it on
#1a1a19 instead — cds gray-830, a warm near-neutral (hue 107) — and retone
the whole dark neutral family to match, since a warm background under cool
panels clashes. Lightness ramp is preserved; chromatic tokens (destructive,
success, icon hues, accent formulas) and the light palette are untouched.

Also introduce --chrome so the app isn't one flat tone: frame strips (tab
bar, status bar, transfer bar, editor/manager header+footer, connection
sidebar) now sit one step below the content panes, with popovers and hover
above them — the standard dark-UI elevation order.

Dark ramp: #151514 chrome, #1a1a19 content, #20201f card, #2e2e2d hover;
the "darker" tone drops chrome to true black. Light gains only a zinc-50
chrome step.

Co-Authored-By: Claude <noreply@anthropic.com>

ui/src/app/tab-bar/tab-bar.ts

@@ -41,7 +41,7 @@ const CONCURRENCY_CHOICES = [2, 3, 4, 6, 8] as const;
     <!-- On macOS this row IS the titlebar (hiddenInset): taller, padded past the
          traffic lights, and draggable. Elsewhere it's a normal compact row. -->
     <div
-      class="relative flex items-center gap-0.5 border-b border-border pr-1.5 electrobun-webkit-app-region-drag"
+      class="relative flex items-center gap-0.5 border-b border-border bg-chrome pr-1.5 electrobun-webkit-app-region-drag"
       [class.h-10]="isMacChrome"
       [class.pl-24]="isMacChrome"
       [class.pl-1.5]="!isMacChrome"