feat(hub): drop offline-mode (HF_HUB_OFFLINE) support

[Mokuroh54]

Running without the Hub is no longer a supported posture. This is a port of the orphaned remove-hf-offline work (original commit b4d752a, written on a pre-rollback lineage) onto current main.

What's removed

• The --offline launcher flag — makermodslab-station is now just --lan; the systemd service file describes the LAN posture (and no longer points at a nonexistent INSTALL.md — pre-existing dangling reference, removed).
• hf_hub_offline() and all nine refuse-when-offline guards (datasets ×5, models ×2, jobs, server training preflight), plus the offline field on /jobs/runners/hardware (all three return paths).
• Frontend: the offline hard-blocks on Start, both CloudNotice offline branches/props, ManageCachesDialog's offline banner + its now-pointless hardware fetch, and the "400 offline" doc comments.

What's deliberately kept

• Every transport-error degrade: hub status "unknown", cached-listing resilience, "offline / rate-limited" prose — those describe network failures, not the removed switch. A grep sweep confirms the only surviving "offline" mentions are this category, local-capability prose, and W&B's unrelated wandb_mode selector.
• 4 non-offline dataset-availability tests from the deleted test_training_preflight.py were relocated into test_datasets.py, not dropped.

Port adaptations (main drifted from the original lineage)

• The old lineage's useTrainingEnvironment/useTrainingConfig hooks don't exist on main; that plumbing is inline in TrainingConfigurator.tsx and was ported there (22-line diff vs the original's 35 — the difference is lineage-only code).
• jobs.py import block adapted to main's shape.

Checks

Backend: ruff clean; scoped pytest 383 passed with zero new failures (7 pre-existing test_models.py isolation failures exist on main tip independently of this PR — real-cache leakage, tracked separately). Frontend: tsc -b and lint identical to main baseline. Full pre-commit run --all-files: green. package-lock.json and frontend/dist/ untouched.

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