feat: add Client Hints support for improved cold-start detection

[superbiche]

Summary

Implements the Client Hints detection proposed in #110, extending the existing utils.ts as suggested by @mvrlin.

• Opt-in via clientHints: true | { viewportWidth, critical } in module config
• Detection chain becomes: cookie → Client Hints → CDN headers → UA → fallback
• Zero behavior change without the clientHints option — fully backward compatible

What's included

• types.ts: new ClientHintsOptions type + clientHints property on ViewportOptions
• utils.ts: Sec-CH-Viewport-Width (pixel-accurate) and Sec-CH-UA-Mobile (default on Chromium) detection, plus resolveBreakpointFromWidth() helper
• plugin.server.ts: sets Accept-CH / Critical-CH response headers when viewportWidth is enabled

Design choices worth noting

This implementation takes a different approach from #112:

• Non-breaking: extends existing detectBreakpoint() instead of rewriting it — no API surface changes, no removed functions
• Breakpoint-aware width resolution: Sec-CH-Viewport-Width maps to configured breakpoints using the minWidth/maxWidth feature mode, rather than a hardcoded threshold
• Respects defaultBreakpoints: Sec-CH-UA-Mobile maps through the existing config (e.g. mobile → "sm") instead of returning raw device type strings
• Full Client Hints lifecycle: includes Accept-CH and Critical-CH response header negotiation, which is required for browsers to send Sec-CH-Viewport-Width
• Preserves dynamic import: keeps Bowser behind import() for chunk splitting

~98 lines added across 3 files.

Test plan

• Verify default behavior unchanged (no clientHints config)
• Enable clientHints: true — confirm Sec-CH-UA-Mobile detection works on Chromium
• Enable clientHints: { viewportWidth: true } — confirm Accept-CH header is set and width-based detection resolves to correct breakpoint
• Enable clientHints: { viewportWidth: true, critical: true } — confirm Critical-CH header is set
• Non-Chromium browsers fall back to UA detection gracefully