fix(frontend): show startup state for pending terminals

[neversettle17-101]

Fixes #2511

Summary

• Keep the existing no-selection terminal placeholder when no session is selected.
• Show a startup message when a selected session exists but does not have a terminal handle yet.
• Add focused coverage for both empty states.

Test

• npm --prefix frontend test -- TerminalPane.test.tsx --run
• npm --prefix frontend run typecheck

Note: typecheck regenerates frontend/src/renderer/routeTree.gen.ts locally in this worktree; that generated churn was intentionally not committed.