fix(web): WCO header alignment, session loading spinner, SPA back link

[setkyar]

Summary

Three related web/PWA fixes:

• WCO header alignment — In Window Controls Overlay (installed PWA), the index header unconditionally added env(titlebar-area-x) to .header-inner's left padding, pushing the logo ~90px right of the content cards on wide windows where the centered max-width: 1040px box already clears the OS controls. Now the inset is only applied when the centered box would actually slide under the window controls (narrow windows), keeping the logo aligned with the cards.
• Session loading spinner — .session-loading had no CSS, so "Loading session…" rendered as bare text in the top-left and flashed away. Replaced with a full-viewport centered layout: a spinner (reusing the existing spin keyframe), a muted label, and a gentle fade-in; the error fallback is styled too. Honors prefers-reduced-motion.
• SPA back link — The session "← Back" link was a plain <a href="/"> triggering a full document reload; under the cross-document view transition that reload could stall in the installed PWA, leaving the homepage stuck on "Loading sessions…". It now routes through handleNavClick like the session cards, navigating client-side while still honoring modified clicks for open-in-new-tab.

Test plan

• Install as PWA in WCO mode: index header logo aligns with the content cards below (wide window); logo still clears the OS window controls on a narrow window.
• Open a session that takes >200ms to load: a centered spinner + label appears (not top-left text), and resolves into the session.
• From a session, click "← Back": homepage loads without getting stuck on "Loading sessions…".
• Cmd/Ctrl-click "← Back" still opens the homepage in a new tab.
• make check passes.