Move Home/Leaderboard nav icons to top bar; persist PoFunQuiz title in header

[Copilot]

Bottom nav was the sole navigation surface, mixing app-level links (Home, Leaderboard) with a debug link (Diag). Moves the primary nav icons to the top header and removes the bottom nav bar entirely.

Changes

• MainLayout.razor — Header now always renders PoFunQuiz title (left) + Home/Leaderboard NavLink icons (right) directly. Removed SectionOutlet and bottom <NavMenu />.
• NavMenu.razor — Stripped to a single Diag top-nav-item rendered only in IsDevelopment().
• Home.razor — Removed the SectionContent HeaderContent block that was redundantly injecting the title.
• app.css — Added global .top-nav / .top-nav-item styles to sidestep Blazor CSS isolation scoping between MainLayout and NavMenu.
• MainLayout.razor.css — Removed padding-bottom: 70px from .app-main; dropped now-unused scoped top-nav rules.
• NavMenu.razor.css — Removed all bottom-nav styles.

<header class="app-header">
    <div class="header-content">
        <span class="app-title" role="button" tabindex="0" @onclick="..." @onkeydown="...">PoFunQuiz</span>
        <nav class="top-nav">
            <NavLink class="top-nav-item" href="" Match="NavLinkMatch.All">
                <span class="material-icons">home</span>
            </NavLink>
            <NavLink class="top-nav-item" href="leaderboard">
                <span class="material-icons">emoji_events</span>
            </NavLink>
            <NavMenu /> <!-- Diag only, dev mode -->
        </nav>
    </div>
</header>

Screenshot

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[codereviewbot-ai]

The property -webkit-overflow-scrolling: touch; is largely obsolete, as modern browsers (including iOS Safari) now provide smooth scrolling by default. Consider removing this property to simplify the stylesheet unless you specifically need to support legacy iOS versions.

Recommendation:

/* Remove this line unless legacy iOS support is required */