Fix empty proxy sources (empty pool), first-run bat, elite hint

- load_settings/sanitize_settings: treat sources=[] as invalid; restore defaults (all([]) was true)
- _build_pool: defensive default sources + clearer empty-fetch log; elite-only empty fetch message
- FirstRun_Build_And_Install.bat: Python check + setup_and_build.ps1
- README + test for empty sources sanitize

Made-with: Cursor

README.md

@@ -62,7 +62,7 @@ Typical flow:
    ```powershell
    powershell -NoProfile -ExecutionPolicy Bypass -File .\scripts\setup_and_build.ps1
    ```
-   Same steps from **`build_exe.bat`** in the repo root (it calls that script).
+   Same steps from **`build_exe.bat`** or double‑click **`FirstRun_Build_And_Install.bat`** (checks Python 3.10+, then runs the same script; pauses at the end so you can read errors).
 
 | Where | What |
 |--------|------|