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/plans/config-settings-window.md

@@ -1,70 +0,0 @@
-# Config Settings Window
-
-read_when: changing config UI, config save behavior, or config docs
-
-## Intent
-
-Add a dedicated Electron settings window for editing canonical config values without exposing the historical layout mistakes in `config.jsonc`.
-
-The UI groups options by workflow:
-
-- Viewing
-- Mining & Anki
-- Playback & Sources
-- Input
-- Integrations
-- Tracking & App
-- Advanced
-
-Each field maps back to its current raw config path. The presentation layer must stay separate from generated config-template sections.
-
-## Sources
-
-- Canonical defaults: `DEFAULT_CONFIG`
-- Existing option descriptions/enums: `CONFIG_OPTION_REGISTRY`
-- UI registry: `src/config/settings/registry.ts`
-- JSONC save path: `src/config/settings/jsonc-edit.ts`
-- Window runtime: `src/main/runtime/config-settings-window.ts`
-
-## Save Contract
-
-Settings writes use `jsonc-parser.modify`, not `JSON.stringify`.
-
-Required behavior:
-
-- Preserve comments, trailing commas, unrelated keys, and hidden legacy keys.
-- Reset removes the explicit path so defaults resolve normally.
-- Validate the candidate config before writing.
-- Reject warnings caused by modified fields.
-- Preserve unrelated existing warnings and return them in the snapshot.
-- Write atomically, reload `ConfigService`, classify with existing hot-reload logic, and apply live changes where supported.
-- Never return secret values to the renderer; snapshots only expose configured/not-configured state.
-
-## Hidden Compatibility Keys
-
-Do not expose these as first-class controls:
-
-- `ankiConnect.deck`
-- Legacy top-level Anki migration fields such as `wordField`, `audioField`, media-generation aliases, and behavior aliases
-- Legacy `ankiConnect.nPlusOne.*` aliases except canonical `nPlusOne.nPlusOne` and `nPlusOne.minSentenceWords`
-- Deprecated Lapis sentence-card fields
-- `youtubeSubgen.primarySubLanguages`
-- `anilist.characterDictionary.refreshTtlHours`
-- `anilist.characterDictionary.evictionPolicy`
-- `jellyfin.accessToken`
-- `jellyfin.userId`
-- `controller.buttonIndices` as a normal editable field
-
-## Verification
-
-Minimum targeted checks:
-
-- `bun test src/config/settings/registry.test.ts src/config/settings/jsonc-edit.test.ts src/settings/settings-model.test.ts src/main/runtime/config-settings-window.test.ts`
-- `bun run test:config`
-- `bun run typecheck`
-- `bun run build`
-
-If docs changed:
-
-- `bun run docs:test`
-- `bun run docs:build`