feat: add mark-watched action, background app reuse, and N+1 compat

- Add `--mark-watched` CLI flag + mpv session binding; marks video watched, shows OSD, advances playlist
- Launcher detects running background app via `--app-ping` and borrows it instead of owning its lifecycle
- Preserve N+1 highlighting for existing configs with `knownWords.highlightEnabled` set
- Fix `resolveConfiguredShortcuts` to respect explicit `null` overrides (disabling defaults)
- Split session-help modal into focused modules (colors, render, sections, tabs)

changes/background-launcher-reuse.md

@@ -0,0 +1,4 @@
+type: fixed
+area: launcher
+
+- Reused an already-running background SubMiner app for launcher-opened videos, preserving warmups and keeping the tray app alive after playback closes.

changes/mpv-session-shortcuts.md

@@ -0,0 +1,4 @@
+type: fixed
+area: overlay
+
+- Wired configured session shortcuts, including `stats.markWatchedKey`, through mpv so custom add/remove changes work while mpv has focus.

changes/nplusone-known-word-compat.md

@@ -0,0 +1,4 @@
+type: changed
+area: config
+
+- Config: Preserved N+1 subtitle highlighting for existing configs that already enabled known-word highlighting, while keeping N+1 disabled by default for new configs unless `ankiConnect.nPlusOne.enabled` is set.