fix: remove bg-black from PageLoader and RouteError, fix PageLoader height

PageLoader used h-screen inside a grid cell, causing it to overflow.
Changed to h-full so it fits within the main-content area. Removed
bg-black from both PageLoader and RouteError in favor of theme-aware
bg-background to prevent black flash during lazy loading.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/common/PageLoader.tsx

@@ -2,7 +2,7 @@ import { Spinner } from '@/components/common/Spinner'
 
 export function PageLoader() {
   return (
-    <div className="flex h-screen items-center justify-center bg-black">
+    <div className="flex h-full items-center justify-center">
       <div className="flex flex-col items-center gap-4">
         <Spinner size="lg" />
         <p className="text-sm text-muted-foreground">Loading...</p>

frontend/src/components/common/RouteError.tsx

@@ -17,7 +17,7 @@ export function RouteError() {
   }
 
   return (
-    <div className="flex min-h-screen flex-col items-center justify-center bg-black p-8">
+    <div className="flex min-h-screen flex-col items-center justify-center bg-background p-8">
       <div className="max-w-md text-center">
         <h1 className="mb-2 text-4xl font-bold text-foreground">Oops!</h1>
         <h2 className="mb-2 text-xl font-semibold text-red-400">{errorMessage}</h2>