web_ui.rs: add available_models() helper that reads HIVE_AVAILABLE_MODELS
(comma-separated, injected by services.hyperhive.availableModels nix option).
Falls back to ["haiku", "sonnet", "opus"] when absent or empty. Field added
to StateSnapshot so the frontend receives the list on cold-load.
app.js: replace hardcoded MODEL_ALIASES array with availableModels module var.
Seeded from state.available_models before setHeader/populateOverflowMenu on
first /api/state load. Well-known aliases (haiku/sonnet/opus) still get their
descriptive labels; operator-declared custom model names show the name itself.
Implements the frontend + web_ui.rs side of issue #1359 (nix option shipped
separately in PR #1360 by atlas).
1f54a07195