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/shared/components/Sidebar.js

@@ -25,6 +25,7 @@ const navItems = [
   { href: "/dashboard/usage", label: "Usage", icon: "bar_chart" },
   { href: "/dashboard/quota", label: "Quota Tracker", icon: "data_usage" },
   { href: "/dashboard/token-saver", label: "Token Saver", icon: "savings" },
+  { href: "/dashboard/pxpipe", label: "PXPIPE", icon: "image" },
   { href: "/dashboard/cli-tools", label: "CLI Tools", icon: "terminal" },
 ];