Multi site: create buildings inline in the Manage Site modal

[flesher]

🤖 Filed by mflesher's AI agent at mflesher's request.

Problem

The Manage Site modal (client/src/protoFleet/features/sites/components/ManageSiteModal/ManageSiteModal.tsx, plus ManageBuildingsModal) lets you assign/associate existing buildings to a site, but there's no way to create a brand-new building without leaving the flow. Operators setting up a site have to bounce out to the Buildings page, create the building, then come back to associate it.

Proposed

Add an inline "Create building" affordance in the Manage Site modal so a new building can be created and attached to the site in one flow.

• Reuse the transactional create-and-seed CreateBuilding RPC (site_id + optional seed) landed in fix(buildings,sites): transactional create-and-seed to prevent orphaned parents (#559) #821 so the building is created and associated atomically — no orphaned parents on failure.
• Mirror the existing inline-create UX where it already exists (e.g. site-create building staging from Multi-site UI hardening round 2: site-create building staging, SitePicker refresh, per-tab counts #561).

Acceptance criteria

• Manage Site modal exposes a "Create building" action alongside assign-existing.
• New building is created against the current site atomically via CreateBuilding.
• Validation + duplicate-name handling consistent with the Buildings page (see Unify duplicate-name allowance & enforcement across sites, buildings, and racks #814).
• Modal list refreshes to show the newly created building.

Related: #264, #715, #814, #821.

[flesher]

🤖 Closing on mflesher's behalf. Shipped: ManageSiteModal now supports inline building creation — onCreateBuilding/onCreateBuildings props, a site-locked inline building-create dropdown, and a single/multiple create toggle (see ManageSiteModal.tsx).