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/connection-manager/connection-manager.html

@@ -2,7 +2,7 @@
   <!-- header: on macOS this row IS the titlebar (hiddenInset + drag region),
        elsewhere the native titlebar sits above and this is a compact strip -->
   <header
-    class="flex shrink-0 items-center gap-2 border-b border-border pr-2"
+    class="flex shrink-0 items-center gap-2 border-b border-border bg-chrome pr-2"
     [class.h-10]="isMacChrome"
     [class.pl-24]="isMacChrome"
     [class.electrobun-webkit-app-region-drag]="isMacChrome"
@@ -29,7 +29,7 @@
 
   <div class="flex min-h-0 flex-1">
       <!-- stacks + connections list -->
-      <div class="flex w-72 shrink-0 flex-col border-r border-border bg-muted/30">
+      <div class="flex w-72 shrink-0 flex-col border-r border-border bg-chrome">
         <div class="flex items-center gap-1 border-b border-border px-2 py-1.5">
           <button
             (click)="newConnection()"
@@ -327,7 +327,7 @@
             }
           </div>
 
-          <div class="flex items-center gap-2 border-t border-border px-4 py-2.5">
+          <div class="flex items-center gap-2 border-t border-border bg-chrome px-4 py-2.5">
             <button
               (click)="deleteCurrent()"
               class="flex items-center gap-1.5 rounded-md px-2 py-1.5 text-xs text-muted-foreground hover:bg-accent hover:text-destructive"