Polish terminal loading labels

[almogdepaz]

Summary

• polish terminal loading/status labels to be shorter and less technical
• add terminalLoadLabelFor() so the label contract has focused unit coverage
• keep terminal state transitions, reveal timing, and animations unchanged

Verification

• bun test tests/unit/terminal-loading-ui.test.ts tests/unit/ghostty-prewarm-pool.test.ts tests/unit/terminal-load-perf.test.ts
• bun run typecheck
• git diff --check
• server-only local deploy with --broker=no
• browser smoke: prefill-loading label observed as loading terminal, final grid cells live, no console errors or failed requests