refactor: externalize tool definitions and polish app shell

- Create lib/tools.tsx as single source of truth for all tool metadata
  (title, shortTitle, navTitle, description, summary, icon, etc.)
- Update AppSidebar to render nav from centralized tools list with
  descriptions, remove collapse footer button
- Update AppHeader with sidebar collapse toggle, tool short title,
  and app logo; remove breadcrumbs
- Update AppPage to auto-resolve tool icon from pathname
- Update ToolsGrid/ToolCard to use shared tools data, remove per-card
  gradients for uniform styling
- Add per-tool HTML title via metadata exports (title template in root
  layout)
- Style landing page and 404 headings with primary theme color
- Add Toolbox icon to hero CTA, GitFork icon link in footer
- Remove footer from error page and "View on Dev" buttons
- Extract ColorPage client component for RSC metadata compatibility

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

components/layout/AppPage.tsx

@@ -1,7 +1,9 @@
 'use client';
 
 import * as React from 'react';
+import { usePathname } from 'next/navigation';
 import { cn } from '@/lib/utils';
+import { getToolByHref } from '@/lib/tools';
 
 interface AppPageProps {
   title: string;
@@ -11,11 +13,19 @@ interface AppPageProps {
 }
 
 export function AppPage({ title, description, children, className }: AppPageProps) {
+  const pathname = usePathname();
+  const firstSegment = pathname.split('/').filter(Boolean)[0];
+  const tool = getToolByHref(`/${firstSegment ?? ''}`);
+  const Icon = tool?.icon;
+
   return (
     <div className={cn("min-h-screen py-8", className)}>
       <div className="max-w-7xl mx-auto px-8 space-y-6 animate-fade-in">
         <div>
-          <h1 className="text-2xl font-bold mb-1">{title}</h1>
+          <div className="flex items-center gap-3 mb-1">
+            {Icon && <Icon className="h-6 w-6 text-primary shrink-0" />}
+            <h1 className="text-2xl font-bold">{title}</h1>
+          </div>
           {description && (
             <p className="text-sm text-muted-foreground">
               {description}