Add FluidNC WASM demo bridge support

[MitchBradley]

Summary

Lets FigUI run against a real, compiled FluidNC instance running in WebAssembly in the browser (see FluidNC/wasm/README.md in the firmware repo), instead of only real hardware or the JS-simulated demo mode. wasmBridge/index.ts's installWasmBridgeIfActive() is called unconditionally from main.tsx (works in every build mode, including the real build:esp32 artifact) and is a no-op unless a runtime marker is present, so it's harmless in a normal deployment — the marker only gets injected when this build is loaded inside the FluidNC WASM demo's iframe.

• wasmBridge/shimTransport.ts — low-level postMessage client talking to the demo page's ShimChannel bridge (raw Grbl-line stream + a file-op request/response RPC).
• wasmBridge/WasmBridgeWebSocket.ts — installed as window.WebSocket so src/lib/ws.ts works unmodified.
• wasmBridge/httpBridge.ts — intercepts fetch/XMLHttpRequest for /command, /upload, /files, etc., matching src/lib/http.ts's real endpoints.
• wasmBridge/commandBridge.ts — request/response command sends for the HTTP-shaped surface.

Since the wasm build shares one physical channel for everything (unlike real hardware's genuinely independent HTTP and WebSocket connections), the last two commits close a couple of races that only show up in that environment:

• JSON reassembly moved to common code: FluidNC's JSONencoder wraps JSON payloads sent over a serial-shaped channel in [JSON:...]-tagged chunks, so a payload line can't collide with the ok/error line that terminates a command. That reassembly used to be done independently in this bridge and in the equivalent bridges for two other WebUI projects (ESP3D-WEBUI, WebUI-mm) that talk to the same demo. It's now done once, by the demo page itself, before anything is ever posted here — this bridge just receives whole, already-unwrapped lines.
• Centralized command queuing: two commands sent through the bridge at once could have their response lines cross-attributed. Fixed by queuing sends centrally on the demo side rather than per-bridge. A real bug surfaced during testing: a raw send made directly through WasmBridgeWebSocket (bypassing the old per-bridge queue) could still race a pending command and steal its ok line, so the queue now serializes all shim traffic — raw sends and RPC commands alike — not just commands against each other.
• Real ESP800 instead of a spoofed response: the wasm build now has an actual [ESP800]json=yes handler (FluidNC/wasm/FwInfo.cpp in the firmware repo) rather than each bridge hardcoding a fake firmware-info response — parser.ts/App.tsx/http.ts updated to consume the real JSON shape.

Note for reviewers

src/lib/http.ts overlaps with e4d3cec (Serialize filesystem ops), already on main. This branch predates that commit, so it doesn't yet reflect it — both efforts independently added serialization to prevent races in their own subsystem (filesystem ops there, shim commands here), just worth being aware of when merging. The overlapping line (getDeviceInfoFast's [ESP800] command string) isn't touched by e4d3cec itself, so I don't expect a real conflict, but flagging it since I haven't rebased onto current main yet.

Test plan

• npx tsc --noEmit clean
• npm run build:esp32 clean
• Verified live against the FluidNC WASM demo: boots, shows live machine status, Settings/Machine Config panels populate from real ESP400/ESP401/ESP800 data, file manager works
• Rebase onto current main and re-verify (12 commits behind at the time of this PR)
• Manual smoke test on real hardware to confirm the bridge is inert (marker check short-circuits) in normal operation

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