Advanced Cross-Platform Audio Player App (Flutter + Firebase) with Dynamic Playlists, Lyrics...

Project Title: Advanced Cross-Platform Audio Player App (Flutter + Firebase) with Dynamic Playlists, Lyrics & Lightweight Social Sharing Project Description: We are looking for an experienced Flutter developer to build a complete, production-ready MVP of a cross-platform audio application (iOS & Android) designed for structured language learning through customized audio playlists. The application must be built using Flutter (frontend) and Firebase (backend: Auth, Firestore, Storage). The architecture must be 100% scalable: all categories, filters, labels, and text must be dynamic (driven by a localization file or remote configuration). Key MVP Features: 1. Multi-Layer Playlist Generator: Step-by-step filtering (Language Pair - Difficulty Level A1-C2 - Topic Selection with Multi-Select - Genre - Playback Mode 1x/2x/3x with or without translation). 2. Ergonomic Player UI & Audio Engine: - UI Layout: Designed for single-handed use (Thumb Zone). All interactive controls (Play/Pause, Skip, Instant Replay, Speed, Likes, Skins) are at the BOTTOM. The scrollable Lyrics text view is in the TOP half. - Audio Engine: Seamless background playback (locked screen/minimized app), integration with system media controls, and an "Instant Replay" button (restarts the current track from 0:00). - Utilities: Audio speed control (0.75x, 1.0x, 1.25x), and a Sleep Timer. 3. Lightweight Anti-Piracy Protection & Viral Sharing: - Social Media Integration (Instagram Stories / TikTok): The app should generate a static visual card image (lyric phrase, translation, app logo) and share it along with a pre-cut 15-second promo audio URL (provided by the backend with a built-in watermark) directly via native Share Sheets / Story Sharing SDKs. No heavy on-device video rendering required. - Messenger Sharing (Telegram, WhatsApp, etc.): Generates a branded dynamic smart-link inviting the recipient to download the app and listen to that specific track (no direct MP3 file sharing). 4. User Personalization & Premium-Ready Architecture: - Like/Dislike system (blacklisting tracks) and "Recently Played" history. - Local database structure (Hive/SQLite) for a future "My Vocabulary" AI feature. - Backend User Schema includes a Boolean flag (is_premium: false) to future-proof the database for subscription paywalls and audio-ad integration. 5. UI Themes (Skins): Support for switching background images and accent colors based on visual assets provided by the client. Project Delivery & Milestones (Fixed Price: €2000 total): - Milestone 1 (€300): Project setup, Localization architecture, and Premium-ready Firebase Database configuration. - Milestone 2 (€400): Playlist Generator UI, multi-layer filtering logic, and Local Vocabulary database setup. - Milestone 3 (€600): Core Audio Engine, Background Playback integration, Speed Control, and Sleep Timer. - Milestone 4 (€400): Ergonomic Player Screen UI, Theme switcher, Likes/Dislikes, and Native Share Sheet / Social Story Integration (Image + Promo Audio URL). - Milestone 5 (€300): Final optimization, end-to-end testing, and successful deployment/approval in the Apple App Store and Google Play Store. Requirements for the Freelancer: - Proven experience with background audio lifecycle in Flutter (just_audio / audio_service packages). - Experience with native share sharing integrations (Social SDKs / Share Sheets). - Strong architecture skills (Clean code, Firebase Firestore/Storage, App Store/Google Play deployment). Please start your proposal with the word "LO-FI" to prove you've read the description. Please provide links to your published apps handling audio streaming.