chore: move all plans to plans folder; add app conversion plan

plans/project plan.md

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+# Project Plan
+
+## Subject Area
+
+- Tool for downloading and synchronizing YouTube playlists.
+- Focuses on batch downloading, format selection (audio and/or video), configurable quality and keeping local copies synced with playlist changes.
+- Targets power users and archivists who need large-scale, repeatable playlist archiving and ongoing synchronization, with GUI interface.
+
+## Problem
+
+- Users and power-users who manage large or frequently changing YouTube playlists lack a dependable, configurable tool that:
+  - correctly detects and downloads new videos while avoiding duplicates,
+  - and can be configured easily via file or GUI for repeatable workflows.
+
+## Users Definition
+
+Individuals who need to download a large number of videos or audio files from a YouTube playlist and keep it updated
+
+## Functionality Definition
+
+- Multi-format Download:
+  - Video only (mp4)
+  - Audio only (mp3)
+  - Both video and audio (mp3 & mp4)
+- Smart Synchronization:
+  - Archive tracking (prevents re-downloading existing media)
+  - Playlist Pruning (automatically deletes local files no longer in the YouTube playlist)
+  - Sequential Renumbering (keeps local files sorted by playlist position)
+- Advanced Configuration:
+  - Per-playlist settings (Quality, paths, archive file)
+  - Global performance options (Parallel downloads, aria2c threading)
+  - Path management for yt-dlp, ffmpeg, and aria2c (Docker-aware)
+- GUI Integration:
+  - Real-time status updates via backend API
+  - Visual configuration editor
+  - Modern, responsive Qt-based interface
+
+## Platforms
+
+- Desktop: Windows (Primary), Linux
+- Docker
+- Possible Future: Web App, Android App (via shared FastAPI backend)
+
+## Architecture & Languages
+
+- Core Engine: Python (yt-dlp wrapper)
+- Backend API: FastAPI (Local localhost-only boundary)
+- Desktop Frontend: PySide6 (Qt for Python)
+- Distribution: PyInstaller / Briefcase (Windows .exe, Linux AppImage)
+