feat(pxpipe): PXPIPE token saver — multimodal prompt compression (#2465)

Add pxpipe as an experimental fifth Token Saver: Claude-format request
bodies above a configurable size threshold are rendered as dense PNGs
via the pxpipe-proxy library API (transformAnthropicMessages) before
dispatch, cutting estimated input tokens by ~35-60% on token-dense
contexts. Integration follows the Headroom pattern: applied to the final
body in chatCore just before dispatch, fail-open on any error/timeout.

Managed npm install into DATA_DIR/pxpipe, dynamic loader with per-version
cache-bust, JSONL event log with rotation, /api/pxpipe/* endpoints, Token
Saver card (marked experimental) + /dashboard/pxpipe page, and per-request
Activated/Skipped annotation in Request Details. Disabled by default.

open-sse/handlers/chatCore/requestDetail.js

@@ -69,6 +69,7 @@ export function buildRequestDetail(base, overrides = {}) {
     providerRequest: base.providerRequest || null,
     providerResponse: base.providerResponse || null,
     response: base.response || {},
+    pxpipe: base.pxpipe || undefined,
     status: base.status || "success",
     ...overrides
   };