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/help.test.ts

@@ -23,6 +23,7 @@ test('printHelp includes configured texthooker port', () => {
   assert.doesNotMatch(output, /--refresh-known-words/);
   assert.match(output, /--setup\s+Open first-run setup window/);
   assert.match(output, /--config\s+Open configuration window/);
+  assert.match(output, /--mark-watched\s+Mark current video watched and advance playlist/);
   assert.match(output, /--anilist-status/);
   assert.match(output, /--anilist-retry-queue/);
   assert.match(output, /--dictionary/);