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)

scripts/test-plugin-session-bindings.lua

@@ -220,6 +220,14 @@ local ctx = {
 						actionType = "mpv-command",
 						command = { "quit" },
 					},
+					{
+						key = {
+							code = "KeyW",
+							modifiers = {},
+						},
+						actionType = "session-action",
+						actionId = "markWatched",
+					},
 					{
 						key = {
 							code = "KeyA",
@@ -307,6 +315,7 @@ local expected_cli_bindings = {
 	{ keys = "Ctrl+Alt+p", flag = "--open-playlist-browser" },
 	{ keys = "Ctrl+H", flag = "--replay-current-subtitle" },
 	{ keys = "Ctrl+L", flag = "--play-next-subtitle" },
+	{ keys = "w", flag = "--mark-watched" },
 }
 
 for _, expected in ipairs(expected_cli_bindings) do