Fix Convex shell environment handling

[nicu-chiciuc]

Why

Convex already supplies VITE_CONVEX_URL: convex dev writes it for local development, and
convex deploy --cmd injects the selected deployment URL into production and preview builds. The
starter instead displayed manual setup instructions when the value was absent, and an
environment-free build could prerender that message as the home page.

The fix must preserve the existing rendering contract: exact / and /about are static generated
pages, while /index.html remains the route-neutral SPA fallback for other app routes.

Change

• mount the Convex provider only around the todo route that uses it
• fail clearly when the real Convex-backed route runs outside a supported dev or deploy command
• preserve exact / at /_landing.html, /about at /about/index.html, and the SPA shell at
  /index.html
• keep the explicit prerender list as the shared source for TanStack Start and the generated
  _redirects block
• document why the Cloudflare-specific root split exists and when it can be reconsidered
• use a CI-only local Convex URL for the discarded GitHub Actions build artifact; development and
  Cloudflare builds still receive the real URL from Convex
• document that Cloudflare needs both Convex deploy keys, but not VITE_CONVEX_URL

Validation

• vp run check
• VITE_CONVEX_URL=http://127.0.0.1:3210 vp run build
• VITE_CONVEX_URL=http://127.0.0.1:3210 CLOUDFLARE_WORKER_NAME=samebase-app vp run deploy:dry-run
• VITE_CONVEX_URL=http://127.0.0.1:3210 CLOUDFLARE_WORKER_NAME=samebase-app vp run deploy:preview:dry-run
• verified dist/client/index.html, dist/client/_landing.html,
  dist/client/about/index.html, and the generated exact redirects
• verified that none of the generated HTML files contains the old manual Convex setup message