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)

src/main/runtime/first-run-setup-service.ts

@@ -90,6 +90,7 @@ function hasAnyStartupCommandBeyondSetup(args: CliArgs): boolean {
     args.triggerSubsync ||
     args.markAudioCard ||
     args.toggleStatsOverlay ||
+    args.markWatched ||
     args.toggleSubtitleSidebar ||
     args.openRuntimeOptions ||
     args.openSessionHelp ||