docs: audit and refresh user-facing and internal docs

Cross-check every config key, shortcut, default, and command against the
current source and fix the drift (mpv.socketPath, auto_start_overlay
default, AniSkip TAB key, JLPT N4 color, secondary-sub font/defaults,
secondary-sub language behavior, modular mpv plugin layout, and more).
Add plain-language intros and first-use definitions across onboarding and
integration pages so non-technical readers can follow along.

Internal docs/: fix stale module paths in architecture/domains.md, add
missing contract entry points and catalog rows, and bump verified dates.
Remove the obsolete docs/plans/ directory (its only plan shipped in
0.15.0) and reframe planning.md so plans live with the work, not in docs/.

docs-site/jellyfin-integration.md

@@ -1,5 +1,11 @@
 # Jellyfin Integration
 
+[Jellyfin](https://jellyfin.org) is a free, self-hosted media server — think of it as your own private streaming service for video you own. If you keep your anime on a Jellyfin server, SubMiner can log in, browse it, and play episodes through mpv with the full mining overlay.
+
+::: tip Who needs this?
+This page is only relevant if you already run (or have access to) a Jellyfin server. If you watch local files or YouTube, you can skip it. Most of this integration is driven from the command line, so it is aimed at slightly more advanced users; the in-app setup window (`subminer jellyfin`) is the easiest starting point.
+:::
+
 SubMiner includes an optional Jellyfin CLI integration for:
 
 - authenticating against a server