chore: rename tool to diffx

[flexdinesh]

• Behavior: rename CLI/server, Go and workspace packages, UI, plugins, docs, Docker and release tooling to diffx.
• Contracts: fresh config/state/producer identities; ingestion protocol 4 and X-Diffx-State preserve destination checks. No aliases or migration.
• Validation: mise run check:push passed, including 63 browser tests and Go race tests; six platform release archives and Docker build/runtime smoke passed. Generated assets clean.
• Architecture/design: remove obsolete Node installer cleanup; release series 0.2 and explicit GoReleaser project name. Repository rename handled separately.
• UI evidence: light/dark, responsive layouts, keyboard interaction and Pierre geometry tests passed.