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-entry-runtime.test.ts

@@ -67,6 +67,40 @@ test('normalizeStartupArgv uses transported AppImage args instead of raw Electro
   );
 });
 
+test('normalizeStartupArgv defaults empty transported AppImage args to background startup', () => {
+  const originalPlatform = process.platform;
+  try {
+    Object.defineProperty(process, 'platform', { value: 'linux', configurable: true });
+
+    assert.deepEqual(
+      normalizeStartupArgv(['SubMiner.AppImage', '--background'], {
+        SUBMINER_APP_ARGC: '0',
+      }),
+      ['SubMiner.AppImage', '--start', '--background'],
+    );
+  } finally {
+    Object.defineProperty(process, 'platform', { value: originalPlatform, configurable: true });
+  }
+});
+
+test('normalizeStartupArgv defaults passive-only transported AppImage args to background startup', () => {
+  const originalPlatform = process.platform;
+  try {
+    Object.defineProperty(process, 'platform', { value: 'linux', configurable: true });
+
+    assert.deepEqual(
+      normalizeStartupArgv(['SubMiner.AppImage'], {
+        SUBMINER_APP_ARGC: '2',
+        SUBMINER_APP_ARG_0: '--password-store',
+        SUBMINER_APP_ARG_1: 'gnome-libsecret',
+      }),
+      ['SubMiner.AppImage', '--password-store', 'gnome-libsecret', '--start', '--background'],
+    );
+  } finally {
+    Object.defineProperty(process, 'platform', { value: originalPlatform, configurable: true });
+  }
+});
+
 test('hasTransportedStartupArgs detects env-carried app args', () => {
   assert.equal(hasTransportedStartupArgs({ SUBMINER_APP_ARGC: '1' }), true);
   assert.equal(hasTransportedStartupArgs({}), false);