Refactor patch management and add ownership safeguards

[xXJSONDeruloXx]

Summary

• Split the Python backend into focused bundle, configuration, Steam discovery, filesystem, and patch-ownership services, leaving main.py as a small Decky façade.
• Keep those runtime services under the template-supported py_modules/services path so Decky includes them in the installable archive and adds them to Python's import path.
• Add schema-2 patch markers with relative target paths, managed-file hashes, verified backups, and conservative legacy cleanup.
• Propagate AppID unpatch cleanup failures instead of reporting success and preserve the marker when cleanup is unsafe.
• Add an explicit global frame-generation backend setting (auto or Nukem's DLSSG → FSR3) and apply it to new/repatched game INIs.
• Abort AppID patching when Steam launch-option APIs cannot safely read/write the existing options.
• Remove the unused commented Installed Games render path and replace the placeholder test command with backend unit tests.

Scope and compatibility

The executable-selection heuristics and existing non-atomic shared-bundle install behavior are intentionally preserved. Top-level bundle uninstall behavior is also unchanged; this PR does not attempt to solve patches remaining after removing ~/fgmod.

Legacy markers and markerless/manual patches are handled conservatively: cleanup removes only files that can be tied to the bundle or an ownership hash, leaves unrelated plugin/D3D12 files in place, and refuses to remove modified managed files.

Validation

• pnpm test — 4 backend safety/configuration tests pass.
• pnpm build — frontend bundle succeeds.
• python3 -m py_compile main.py py_modules/services/*.py tests/test_backend.py.
• bash -n defaults/assets/fgmod.sh defaults/assets/fgmod-uninstaller.sh.
• .vscode/build.sh — full Decky package build succeeds.
• out/Decky-Framegen.zip passes unzip -t, contains the expected root metadata, dist/, bin/, runtime assets/, and py_modules/services/, and has no backend/, node_modules, .git, out, or __pycache__ artifacts.
• Import smoke test passes from the extracted archive using Decky's py_modules layout.
• Containerized Decky package build was previously attempted but could not run because the local Docker daemon was unavailable; the full build now succeeds with Docker available.