Add responsive breakpoints for mobile layout

[bonheur15]

Problem

The landing page has no @media queries anywhere in the CSS. On narrow screens, the layout breaks.

Location: CSS in internal/web/views/layout.templ

Specific Issues

1. 3-column form row (.builder .row, line 76-77): Width/Quality/Format inputs are display: flex with no flex-wrap — they become cramped on screens < 400px
2. Tables (lines 123-125): Options table and benchmark table have no responsive handling — content overflows on mobile
3. Tabs (.tabs, line 37): Tab labels don't wrap on narrow screens
4. Main padding (line 96): Fixed 4rem 1.25rem padding — excessive on mobile
5. Builder padding (line 60): Fixed 1.25rem 1.5rem 1.5rem — no mobile reduction

Proposed Fix

Add at minimum a single mobile breakpoint:

@media (max-width: 500px) {
    main {
        padding: 2rem 1rem 4rem;
    }
    
    .builder .row {
        flex-direction: column;
        gap: 0.5rem;
    }
    
    .tabs {
        flex-direction: column;
    }
    
    table {
        font-size: 0.8rem;
    }
    
    th, td {
        padding: 0.4rem 0.6rem;
    }
}

Why This Matters

The image CDN builder is a key onboarding tool. If it's unusable on mobile, users testing the service from their phone get a poor first impression.