Refactor GUI and CLI onto a shared compression core

[SLP-DEV1]

Summary

Remove duplicated Sharp compression logic from the server and CLI and move it into one shared Node compression engine.

Shared core

• add lib/compression-core.mjs
• centralize format normalization and extension mapping
• centralize Sharp pipeline construction
• centralize resize, metadata, JPG flatten, PNG/WebP/JPEG encoder settings
• centralize target-size binary quality search
• centralize Auto Best candidate generation and selection
• centralize candidate summaries and metadata reads

Server

• keep API/job/reSmush/ZIP behavior in server/index.mjs
• delegate all local compression, Auto Best and target-size work to the shared core
• remove the previous duplicate implementations

CLI

• delegate local, Auto Best and target-size processing to the same shared core
• preserve existing CLI-facing method names and JSON structure through a thin adapter
• keep CLI local-only

CI

• syntax-check the shared core directly
• exercise target-size and Auto Best directly against an in-memory generated image
• keep the existing end-to-end CLI target-size and Auto Best tests
• keep strict TypeScript and production build validation

The goal is behavioral consistency: future encoder or search changes should be implemented once instead of separately in GUI/server and CLI.