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/demos.md

@@ -1,6 +1,6 @@
 # Feature Demos
 
-Short recordings of SubMiner's key features and integrations from real playback sessions.
+Short recordings of SubMiner's key features and integrations from real playback sessions. A few terms you'll see below: _Yomitan_ is the pop-up dictionary used for word lookups, _Jimaku_ is a community subtitle database, _alass_ and _ffsubsync_ are tools that retime subtitles to match the audio, _Jellyfin_ is a self-hosted media server, and a _texthooker_ is a web page that mirrors the current subtitle as selectable text for browser-based tools.
 
 <script setup>
 import { withBase } from 'vitepress';