refactor(web): unify max-width constraints with tailwind content class

Adds maxWidth.content: '720px' to tailwind.config.ts for centralized width management across the web app. Replaces all hardcoded max-w-[720px] references with the new max-w-content class. Adds missing max-width constraints to file.tsx, files.tsx, and SessionList.tsx. Moves border-b dividers from outer full-width divs to inner max-w-content divs for consistent visual hierarchy.

web/src/components/SessionHeader.tsx

@@ -50,7 +50,7 @@ export function SessionHeader(props: {
 
     return (
         <div className="bg-[var(--app-bg)] pt-[env(safe-area-inset-top)]">
-            <div className="mx-auto w-full max-w-[720px] flex items-center gap-2 p-3">
+            <div className="mx-auto w-full max-w-content flex items-center gap-2 p-3">
                 {/* Back button */}
                 <button
                     type="button"