refactor(frontend): extract SHOWCASE_ROUTE_PREFIX constant

[icn5381]

Replace the two hardcoded /showcase/ literals (the URL builder in static-demo.ts and the mock-mode path check in use-thread-chat.ts) with a shared SHOWCASE_ROUTE_PREFIX constant exported from static-demo.ts, so the route dependency is explicit and grep-able rather than scattered.

Suggested in the #4635 review.

References #4635

Fixes #

Why

What changed

Surface area

• Frontend UI — page / component / setting / interaction under frontend/
• Backend API — endpoint / SSE event / request-response shape under backend/app
• Agents / LangGraph — agent node, graph wiring, langgraph.json, or prompt change
• Sandbox — docker/ or sandboxed execution
• Skills — change under skills/
• Dependencies — new/upgraded entry in backend/pyproject.toml or frontend/package.json (say what it buys us)
• Default behavior change — changes existing behavior without the user opting in (default model, default setting, data shape)
• Docs / tests / CI only — no runtime behavior change

Screenshots / Recording

Bug fix verification

Validation

AI assistance

Tool(s) used:

How you used it:

• I've read and understand every line of this change and take responsibility for it — it's not unreviewed AI output.