docs: trust-boundary image on Overview

[LukasWodka]

Replace the Mermaid trust-boundary diagram on the Environment Setup Overview with the federated-learning image (/images/traceblocfederatedlearning.png), matching /overview/tracebloc.

[LukasWodka]

👋 Heads-up — Code review queue is at 14 / 8

Above the WIP limit. The team convention is to review existing PRs before opening new work.

Open PRs currently in Code review (oldest first):

• averaging-service#102 — chore(kanban): declare staging as prod-truth branch via .kanban.yml · author: @LukasWodka · no reviewer assigned
• averaging-service#103 — fix(averaging): correctness foundations — DP, shape guard, sklearn weighting, bounded ensembles (#73) · author: @LukasWodka · reviewer: @shujaatTracebloc
• averaging-service#98 — ci: add fr-gate caller workflow · author: @LukasWodka · no reviewer assigned
• backend#732 — feat(experiments): add per-experiment aggregation_strategy field (#730) · author: @LukasWodka · reviewer: @saqlainsyed007, @shujaatTracebloc
• data-ingestors#146 — test(schema): lock category enum to the engine's TaskCategory (anti-drift) · author: @LukasWodka · reviewer: @saadqbal
• data-ingestors#148 — Develop to prod · author: @divyasinghds · reviewer: @saadqbal
• design-system#19 — fix: un-track coverage/ and node_modules/ from git · author: @LukasWodka · no reviewer assigned
• design-system#22 — ci: add Vitest test workflow · author: @LukasWodka · reviewer: @aptracebloc
• design-system#23 — ci: add fr-gate caller workflow · author: @LukasWodka · no reviewer assigned
• docs#47 — docs(environment-setup): #192 namespace/workspace flow (hold for client#192) · author: @LukasWodka · reviewer: @saadqbal

Pull from review before opening new work. (This is a nudge from the kanban WIP check, not a block.)