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.

src/sse/handlers/chat.js

@@ -12,6 +12,8 @@ import { getSettings } from "@/lib/localDb";
 import { getModelInfo, getComboModels } from "../services/model.js";
 import { handleChatCore } from "open-sse/handlers/chatCore.js";
 import { DEFAULT_HEADROOM_URL } from "@/lib/headroom/detect";
+import { getTransform as getPxpipeTransform } from "@/lib/pxpipe/loader.js";
+import { appendPxpipeEvent } from "@/lib/pxpipe/events.js";
 import { errorResponse, unavailableResponse } from "open-sse/utils/error.js";
 import { handleComboChat, handleFusionChat } from "open-sse/services/combo.js";
 import { handleBypassRequest } from "open-sse/utils/bypassHandler.js";
@@ -259,6 +261,12 @@ async function handleSingleModelChat(body, modelStr, clientRawRequest = null, re
       cavemanLevel: chatSettings.cavemanLevel || "full",
       ponytailEnabled: !!chatSettings.ponytailEnabled,
       ponytailLevel: chatSettings.ponytailLevel || "full",
+      pxpipeEnabled: !!chatSettings.pxpipeEnabled,
+      pxpipeMinChars: chatSettings.pxpipeMinChars,
+      pxpipeTimeoutMs: chatSettings.pxpipeTimeoutMs,
+      // Lazily warms the in-process module on first use; null when not installed (fail-open)
+      pxpipeTransform: chatSettings.pxpipeEnabled ? await getPxpipeTransform() : null,
+      onPxpipeEvent: appendPxpipeEvent,
       providerThinking,
       // Detect source format by endpoint + body
       sourceFormatOverride: request?.url ? detectFormatByEndpoint(new URL(request.url).pathname, body) : null,