feat(annotate): annotate live localhost design previews

[notxcain]

Summary

Adds plannotator annotate http://localhost:PORT/path — renders a live frontend dev-server page (as an HTML artifact) for annotation, instead of converting it to markdown.

Partially addresses #642 (annotate a live website/URL in a frame, no extension, works over SSH) — scoped here to localhost/loopback dev previews; arbitrary public URLs still convert to markdown.

• Per-session reverse proxy forwards to the dev server, proxies its HMR websocket (no reload loop), and injects the annotation bridge into HTML responses. Loopback-only (isLoopbackUrl guard), bound to 127.0.0.1. Bun impl (packages/server/preview-proxy.ts) + node:http mirror for Pi (apps/pi-extension/server/previewProxy.ts).
• Editor renders an <iframe src> at the proxy origin (additive optional src prop on HtmlViewer) so the real ES-module graph loads with no null-origin CORS problem; annotation works via the existing postMessage bridge. The srcDoc path is byte-for-byte unchanged when src is absent (@plannotator/ui seam rule).
• Threads livePreviewUrl through the annotate /api/plan (Bun, OpenCode, Pi) and CLI dispatch, carrying the target path onto the iframe URL.
• Live-preview sessions disable URL sharing, version history, and host-theme token injection.

Why a proxy (not <base> / srcdoc)

A srcDoc iframe is a null origin; a modern Vite dev server returns no Access-Control-Allow-Origin to a null origin, so its module scripts are CORS-blocked → blank page. And its URLs are root-absolute (/@vite/client, /src/main.tsx), which a <base> can't redirect. A proxy that owns the origin root solves both.

Caveat for reviewers — Pi runtime not fully verified

The Pi runtime's live-preview path (apps/pi-extension/server/previewProxy.ts, a node:http mirror of the Bun proxy) is not end-to-end verified. HTTP forwarding + bridge injection were smoke-tested against a real dev server, but the HMR-websocket passthrough was never exercised against a live HMR client — no Pi runtime was available this session. It is a dependency-free raw-socket implementation mirroring the Bun version. One known edge case: the WS passthrough hooks the socket connect event, which is correct for plain TCP but wrong for TLS — an https-loopback dev server with HMR would need secureConnect (plain-http loopback, the common case, is unaffected). The Claude Code path is verified end-to-end in a browser; OpenCode shares the same server/UI code but was only build-checked.