chore(docs): implement documentation

docs/user/configuration.md

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+## Configuration
+
+Open Configuration in the web UI. Tabs:
+
+- General (admin)
+  - App version, basic info
+- Downloads (admin)
+  - Concurrent downloads, retry behavior
+  - Quality/format defaults and conversion
+  - Real-time mode (Spotify only): aligns download time with track length
+- Formatting (admin)
+  - File/folder naming patterns (examples)
+    - `%artist%/%album%/%tracknum%. %title%`
+    - `%ar_album%/%album% (%year%)/%title%`
+- Accounts (admin)
+  - Spotify: use `spotizerr-auth` to add credentials
+  - Deezer ARL (optional):
+    - Chrome/Edge: devtools → Application → Cookies → https://www.deezer.com → copy `arl`
+    - Firefox: devtools → Storage → Cookies → https://www.deezer.com → copy `arl`
+    - Paste ARL in Accounts
+  - Select main account when multiple exist
+- Watch (admin)
+  - Enable/disable watch system
+  - Set check intervals
+  - Manually trigger checks (artists/playlists)
+- Server (admin)
+  - System info and advanced settings
+- Profile (all users when auth is enabled)
+  - Change password, view role and email
+
+Quality formats (reference):
+- Spotify: OGG 96k/160k/320k (320k requires Premium)
+- Deezer: MP3 128k/320k (320k may require Premium), FLAC (Premium)
+- Conversion: MP3/FLAC/AAC/OGG/OPUS/WAV/ALAC with custom bitrate
+
+Fallback system:
+- Configure primary and fallback services
+- Automatically switches if primary fails (useful for geo/account limits)
+
+Notes:
+- Explicit content filter applies in pages (e.g., hides explicit tracks on album/playlist views)
+- Watch system must be enabled before adding items