style: align header, content, and footer widths

Standardized all layout sections to use consistent max-width:
- Changed from inconsistent container/max-w-6xl to unified max-w-7xl
- Header: container mx-auto → w-full max-w-7xl mx-auto
- Main: container mx-auto → w-full max-w-7xl mx-auto
- Footer: container mx-auto → w-full max-w-7xl mx-auto
- MainConverter: removed max-w-6xl (inherits from parent)

All sections now align perfectly with matching left/right edges
for a clean, professional layout.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

components/converter/MainConverter.tsx

@@ -138,7 +138,7 @@ export default function MainConverter() {
   }, [selectedUnit]);
 
   return (
-    <div className="w-full max-w-6xl mx-auto space-y-6">
+    <div className="w-full space-y-6">
       {/* Command Palette */}
       <CommandPalette
         onSelectMeasure={setSelectedMeasure}

components/ui/Footer.tsx

@@ -5,7 +5,7 @@ export default function Footer() {
 
   return (
     <footer className="border-t mt-16 bg-background">
-      <div className="container mx-auto px-4 py-8">
+      <div className="w-full max-w-7xl mx-auto px-4 py-8">
         <div className="grid grid-cols-1 md:grid-cols-3 gap-8">
           {/* About */}
           <div>