fix(desktop): make guided setup reliable for fresh installations

[tulayha]

Summary

• replace the non-descriptive managed-setup spinners with a blocking progress dialog that reports the active installation stage
• isolate native dependency import probes, exclude model downloads from staged-runtime validation, and preserve structured failure output alongside native diagnostics
• embed the exact validated VidXP wheel in every Desktop installer while resolving its selected, constrained dependencies from production PyPI
• pass the already-tested Python artifact into pull-request and release-candidate Desktop builds so an unpublished beta can complete a fresh setup
• document the bundled bootstrap contract; independent managed-runtime updates remain tracked in Decouple Desktop and managed runtime releases with a signed compatibility manifest #105

This requires no repository rebuild, data migration, or new user configuration. Existing active runtimes remain unchanged unless the replacement passes validation.

Validation

• npm --prefix desktop run check — TypeScript typecheck, ESLint, 38 Vitest component/adapter tests, and production frontend build
• rustup run 1.97.1 cargo test --locked --manifest-path desktop/src-tauri/Cargo.toml — 79 Rust unit/process-contract tests with the candidate wheel embedded by the build script
• cargo fmt --manifest-path desktop/src-tauri/Cargo.toml -- --check
• uv run --no-sync ruff check .
• uv run --no-sync python -m pytest tests/test_cli.py tests/test_capabilities.py -q — 52 targeted CLI and capability tests
• uv run --no-sync python -m unittest discover -s tests -q — 606 repository CI-style tests, 4 skipped
• verified the built vidxp-0.4.0b3 wheel installs locally with --no-deps, then resolves the scene extra from production PyPI in a uv pip install --dry-run
• Actionlint 1.7.12 and YAML parsing accepted all workflow files
• git diff --check

The complete 674-test pytest invocation did not finish within the five-minute local Windows command cap and reported no failing test before termination; the repository's CI-style unittest gate completed successfully. No macOS installer was built or manually exercised on this Windows host.