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)

plugin/subminer/session_bindings.lua

@@ -151,6 +151,8 @@ function M.create(ctx)
 			return { "--toggle-subtitle-sidebar" }
 		elseif action_id == "markAudioCard" then
 			return { "--mark-audio-card" }
+		elseif action_id == "markWatched" then
+			return { "--mark-watched" }
 		elseif action_id == "openRuntimeOptions" then
 			return { "--open-runtime-options" }
 		elseif action_id == "openJimaku" then