fix(sites): plain centered empty state for site buildings pane

[flesher]

What & why

The Manage Site modal's right pane renders each building as a fixed-size gray FPO tile (PlaceholderBlock). When a site had no buildings, the empty state reused that same tile treatment — producing a bordered white box that read as a building and forced "No buildings in this site" to wrap inside the 120px width.

This swaps that one branch for plain text centered on both axes in the existing flex container, with whitespace-nowrap to stop the wrap. The empty state now reads as an empty state rather than a stray building card.

Scope

The loading and populated states still use PlaceholderBlock — those remain FPO pending the real BuildingCard component (#263). Non-goal / deferred: no change to the tile design itself or the broader FPO-to-real-component migration; this is a targeted empty-state cleanup only.

🤖 Generated with Claude Code

[github-actions]

🔐 Codex Security Review

Note: This is an automated security-focused code review generated by Codex.
It should be used as a supplementary check alongside human review.
False positives are possible - use your judgment.

Scope summary

• Reviewed pull request diff only (1eab1dbe2bed41b1c1b19ca125b507f70d3dfe39...7f35561f25ded08f0cde2d493bb038695e813050, exact PR three-dot diff)
• Model: gpt-5.6-sol

💡 Click "edited" above to see previous reviews for this PR.

---

Review Summary

Overall Risk: NONE

Findings

No security, correctness, or reliability findings.

Notes

The diff only replaces a static empty-state placeholder with static React text. It does not introduce untrusted input handling, authentication changes, network behavior, persistence, concurrency, or command execution.

---

_{Generated by Codex Security Review |
Triggered by: @flesher |
Review workflow run}

[Copilot]

Pull request overview

Updates the ProtoFleet Manage Site modal’s right-hand “buildings preview” pane to render a clearer empty state when a site has no buildings, avoiding the placeholder-tile styling that made the empty state look like a building card and forced awkward wrapping.

Changes:

• Replace the “no buildings” preview state from a fixed-size PlaceholderBlock tile to centered plain text.
• Add whitespace-nowrap to keep the empty-state message on one line.