Per-site settings overrides

[mxmgorin]

Let selected settings be overridden per website, layered over the global config for the current page. Build the host-keyed plumbing once; each setting opts in.

Design

• Host-keyed override config (e.g. [sites."example.com"] holding a subset of keys; missing keys fall back to global)
• Resolve by navigated host and layer over the global config for that page
• Add/edit a site override in the UI (page menu / settings sub-view)

Overridable settings

• Experimental features (first) — preset / per-feature; [experimental] already shaped to grow a sites sub-table; reuses engine::set_experimental_prefs on navigation
• User agent — force desktop/mobile per site (global pref → re-apply on navigation)
• Page zoom — per-site zoom (per-webview set_page_zoom)
• Ad blocker on/off — per-site allowlist (delegate sees each load's host)
• Data saving — per-site block_images/media/fonts + max_images_per_page (delegate host check)
• Record history — per-site "don't log" (maybe)

Notes: per-webview settings (zoom) and per-load host checks (adblock, data-saving) are natural per-site; global Servo prefs (UA, experimental) re-apply on navigation.