fix(web): stop horizontal overflow on mobile landing sections

[hmziqagent]

Problem

Several components on the homepage caused horizontal overflow on mobile — the whole page was ~622px wide, forcing horizontal scroll at 390/360/320px.

Root cause

Three landing grids use grid lg:grid-cols-[…] without a base grid-cols-1, so the implicit column sized to max-content and wide children (a long tab label, a long code line, the event-log panel) inflated the entire grid past the viewport.

Changes

• landing/v1/HookTabs.astro — add base grid-cols-1; make the tab <nav> scroll horizontally (min-w-0 max-w-full overflow-x-auto) and stop the buttons compressing (shrink-0 whitespace-nowrap). The nav is a flex item of Basecoat's .tabs, so min-w-0 is required for overflow-x-auto to actually engage. (card was 602px → now 358px)
• landing/v3/AnnotatedFigure.astro — add base grid-cols-1 so the <pre> scrolls instead of inflating its card. (529px → 390px)
• landing/v2/CacheDeck.astro — add base grid-cols-1 so the event-log panel fits. (overflowed ~72px @320)
• landing/architecture-section.astro — truncate → sm:truncate so the layer API list wraps on mobile instead of being ellipsized away (still truncates ≥640px).

Verification

Rendered the homepage in a headless browser at 390 / 360 / 320px:

• document.scrollWidth now equals the viewport at every width (was 622) — hasHScroll: false.
• Per-component widths back in bounds (HookTabs 602→358, AnnotatedFigure 529→390, CacheDeck internal 376→362).
• Architecture items now wrap to 2 readable lines instead of clipping.
• bun run build passes.

Desktop (lg) layouts are unchanged — every fix only affects the mobile base layer or is inert when content already fits.

Note: PR is from a fork (hmziqagent/gpui-query) because the authed account has read-only access to freeoxide/gpui-query.