docs: add audit of current app state to README (FR)

[MLyte]

Motivation

• Provide an up-to-date, honest snapshot of the repository status and runtime constraints so contributors and users understand what works today and what remains to do.
• Document verification results observed in the current environment (tests, CLI, FFmpeg availability, offline install issues) to avoid confusion during onboarding.

Description

• Rewrote and extended README.md (French) to include an "Audit de l'état actuel (22 avril 2026)" section listing implemented features, partially implemented items, risks, and short-term priorities, and clarified installation notes regarding offline environments and ffmpeg on PATH.
• Kept usage/quick-start, outputs, and scope sections aligned with the audit and clarified that the GUI requires PySide6; no application logic or source code was modified.

Testing

• Ran unit tests with PYTHONPATH=src python -m pytest -q, resulting in 4 passed.
• Verified CLI help with PYTHONPATH=src python -m optimaster --help, which succeeded.
• Verified PYTHONPATH=src python -m optimaster presets, which listed built-in presets successfully.
• ffmpeg -version failed in the audit environment because ffmpeg is not installed there.
• python -m pip install -e . failed in the audit environment due to offline/proxy preventing dependency resolution.

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Codex Task