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/cli/args.test.ts

@@ -94,6 +94,7 @@ test('parseArgs captures youtube startup forwarding flags', () => {
 test('parseArgs captures session action forwarding flags', () => {
   const args = parseArgs([
     '--toggle-stats-overlay',
+    '--mark-watched',
     '--open-jimaku',
     '--open-youtube-picker',
     '--open-playlist-browser',
@@ -110,6 +111,7 @@ test('parseArgs captures session action forwarding flags', () => {
   ]);
 
   assert.equal(args.toggleStatsOverlay, true);
+  assert.equal(args.markWatched, true);
   assert.equal(args.openJimaku, true);
   assert.equal(args.openYoutubePicker, true);
   assert.equal(args.openPlaylistBrowser, true);
@@ -285,6 +287,12 @@ test('hasExplicitCommand and shouldStartApp preserve command intent', () => {
   const toggleStatsOverlayRuntime = parseArgs(['--toggle-stats-overlay']);
   assert.equal(commandNeedsOverlayRuntime(toggleStatsOverlayRuntime), true);
 
+  const markWatched = parseArgs(['--mark-watched']);
+  assert.equal(markWatched.markWatched, true);
+  assert.equal(hasExplicitCommand(markWatched), true);
+  assert.equal(shouldStartApp(markWatched), true);
+  assert.equal(commandNeedsOverlayRuntime(markWatched), true);
+
   const dictionary = parseArgs(['--dictionary']);
   assert.equal(dictionary.dictionary, true);
   assert.equal(hasExplicitCommand(dictionary), true);