feat: New Fleet wizard Step 2 rewrite — vendor + chat platform + ACP (studio#128)

[brettchien]

Summary

Items 1-5 of #128's runbook, shipped together (all fields on the same new Step 2 screen — not worth splitting further). Replaces the Template/Overlay compose-library step with the vendor + chat platform + ACP flow confirmed with Brett across #128's design thread.

This directly fixes the original problem that started #119/#128: the Template dropdown ships empty on every fresh install (no bundled default, by design) and blocked the whole "+ New fleet" flow.

Console — new Step 2

• Vendor <select>: Claude / codex / agy / cursor / kiro. "agy" is only the display label — the option value is "antigravity" (the actual GHCR image variant / Dockerfile.package build-target name resolve_vendor_image_tags expects). Caught this via a real typecheck failure (unused VENDORS translation table) before it shipped — simpler fix was making the HTML option value correct directly, not translating at call time.
• Image tag: pre-filled from resolve_vendor_image_tags (feat: resolve vendor Stable/Beta image tags from GHCR (studio#128) #131) — Stable if GHCR confirms it exists, else Beta (pre-beta-<vendor>), else left for manual entry. Always editable, a lookup failure never blocks the field.
• API key (optional): captured, stored as a secret; not yet wired into config.toml (documented gap carried over from feat(studio-cp,oab-mcp,src-tauri): add deploy_provision_agent (studio#128) #129).
• Chat platform (optional, defaults to none — "use ACP directly"): Discord/Telegram/LINE, reveals the right token field(s) per platform. k8s deploys refuse a non-empty selection both client-side (clean validation message before even calling the tool) and server-side (feat(studio-cp,oab-mcp,src-tauri): add deploy_provision_agent (studio#128) #129's existing refusal — belt and suspenders, not redundant: the client check gives a better message, the server check is the real guarantee).
• Enable ACP checkbox: default checked, forced off + disabled when vendor = agy (its bridge bypasses openab-gateway's /acp route entirely — confirmed by reading agy-acp/src/main.rs).
• Agent name: pre-filled with a random Greek god name (Brett's request), shuffle button to re-roll, still freely editable.
• No separate "Preview bundle" step anymore — fill in the fields, Deploy.

Backend: acp_enabled made caller-controlled

#127 hardcoded acp_enabled: Some(true) inside build_default_manifest/build_default_k8s_manifest with no way for a caller to turn it off — fine when nothing exposed a toggle yet, not fine once the wizard has an actual checkbox (needed for the agy case above, which has to be forceable to false).

• studio-cp: both functions gained an acp_enabled: bool parameter — only generates the ACP auth secret (AWS Secrets Manager / k8s Secret) when true, no wasted secret nobody will reference. provision_from_library/provision_from_library_k8s (the older compose-library path — unrelated to this wizard, still exists) pass true at their call sites, preserving feat(oabctl,studio-cp): enable ACP by default on Studio-deployed agents #127's original unconditional-on behavior there, unchanged.
• oab-mcp: deploy_provision_agent's schema gained acp_enabled: boolean, defaulting to true server-side when the caller omits it (same default feat(oabctl,studio-cp): enable ACP by default on Studio-deployed agents #127 had, now explicit and overridable instead of baked in).
• src-tauri: bridge command threads the new param through.

Verification

• npm run typecheck clean, npm test 106/106 passing, npm run build succeeds.
• Rust side not locally compiled — same pre-existing aws-sdk-ec2 OOM limitation as every other Rust PR this week.
• Not visually/interactively checked in a running app — this is the biggest UI change of the batch, genuinely worth Brett clicking through once merged: New Fleet → pick each vendor (check agy forces ACP off) → pick each chat platform (check the right token fields appear/disappear) → Deploy on both AWS and k8s.

Ref #128.

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