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.ts

@@ -4655,6 +4655,7 @@ function compileCurrentSessionBindings(): {
     keybindings: appState.keybindings,
     shortcuts: getConfiguredShortcuts(),
     statsToggleKey: getResolvedConfig().stats.toggleKey,
+    statsMarkWatchedKey: getResolvedConfig().stats.markWatchedKey,
     platform: resolveSessionBindingPlatform(),
     rawConfig: getResolvedConfig(),
   });
@@ -5141,6 +5142,17 @@ async function dispatchSessionAction(request: SessionActionDispatchRequest): Pro
     toggleSecondarySub: () => handleCycleSecondarySubMode(),
     toggleSubtitleSidebar: () => toggleSubtitleSidebar(),
     markLastCardAsAudioCard: () => markLastCardAsAudioCard(),
+    markActiveVideoWatched: async () => {
+      const marked = (await appState.immersionTracker?.markActiveVideoWatched()) ?? false;
+      if (marked) {
+        try {
+          await maybeRunAnilistPostWatchUpdate({ force: true });
+        } catch (error) {
+          logger.warn('Failed to run AniList post-watch update after manual watched mark:', error);
+        }
+      }
+      return marked;
+    },
     openRuntimeOptionsPalette: () => openRuntimeOptionsPalette(),
     openJimaku: () => openJimakuOverlay(),
     openSessionHelp: () => openSessionHelpOverlay(),
@@ -5162,6 +5174,8 @@ async function dispatchSessionAction(request: SessionActionDispatchRequest): Pro
         (text) => showMpvOsd(text),
       );
     },
+    playNextPlaylistItem: () =>
+      sendMpvCommandRuntime(appState.mpvClient, ['playlist-next', 'force']),
     showMpvOsd: (text) => showMpvOsd(text),
   });
 }