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/development.md

@@ -68,10 +68,15 @@ make dev-watch-macos                          # same as dev-watch, forcing --bac
 ```
 
 For mpv-plugin-driven testing without exporting `SUBMINER_BINARY_PATH` each run, set a one-time
-dev binary path in `~/.config/mpv/script-opts/subminer.conf`:
+dev binary path with `mpv.subminerBinaryPath` in your SubMiner config. The launcher injects it into
+the mpv plugin at runtime:
 
-```ini
-binary_path=/absolute/path/to/SubMiner/scripts/subminer-dev.sh
+```json
+{
+  "mpv": {
+    "subminerBinaryPath": "/absolute/path/to/SubMiner/scripts/subminer-dev.sh"
+  }
+}
 ```
 
 ## Testing