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/installation.md

@@ -1,5 +1,7 @@
 # Installation
 
+SubMiner is a desktop app that draws an interactive layer — an **overlay** — on top of the [mpv](https://mpv.io) video player. As you watch native Japanese media, you can click or hover any word in the subtitles to look it up, then turn it into an Anki flashcard without pausing to switch apps. Building flashcards from real content you're watching is called **sentence mining**, and it's what SubMiner is built for. It bundles its own copy of **Yomitan** (a pop-up dictionary) and talks to **AnkiConnect** (an add-on that lets other programs add cards to Anki) so cards get filled in automatically.
+
 Three steps to get started:
 
 1. **Install requirements** — mpv and a few optional extras
@@ -92,7 +94,7 @@ pip install ffsubsync
 
 ### macOS
 
-macOS 10.13 or later. Accessibility permission is required for window tracking (see [step 2](#macos-dmg)).
+macOS 11 (Big Sur) or later. Accessibility permission — the macOS setting that lets one app observe and position another app's windows — is required so the overlay can follow the mpv window (see [step 2](#macos-dmg)).
 
 ```bash
 brew install mpv ffmpeg