Add HTTP interface, ZPL/GRF flash memory simulation, and Labelary rate-limit protection

[vinceformica-cyncly]

Summary

• HTTP interface — embedded web server (configurable port, default 9200) with a ZPL template editor and live label preview. Preview uses the same server-side pipeline as TCP (^DF + ^XF sent to Labelary natively), eliminating any client-side field substitution discrepancy.
• ^DF / ^XF flash memory — format templates downloaded via ^DF are persisted to disk and recalled via ^XF. On recall, the stored template is prepended as a ^DF label so Labelary resolves the ^XF natively — identical behaviour to real Zebra hardware.
• ~DG / ^XG flash memory — GRF image blobs uploaded via ~DG are stored and injected inline before any ^XG reference is sent to Labelary.
• Labelary rate-limit protection — a SemaphoreSlim(1,1) serializes all Labelary calls across concurrent TCP connections. Semaphore acquisition happens at the start of each job so processing order matches TCP acceptance order. A configurable inter-request delay (default 400 ms, exposed in Global Settings) keeps burst workloads within the free-tier 3 req/sec limit.

Test plan

• Send a single ZPL label via TCP — renders correctly
• Send a 5-unit work order — all 5 labels render in sequence (1,2,3,4,5) with no HTTP 429 errors
• Send ~DG image blob then a label with ^XG reference — image appears in rendered label
• Send ^DF template then a print job with ^XF recall — label renders with field values populated
• Open browser editor, set field values, click Preview — label renders matching TCP output
• Change Labelary Request Delay in Global Settings — saved and applied on next job