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.ts

@@ -39,6 +39,7 @@ ${B}Mining${R}
   --trigger-field-grouping       Run Kiku field grouping
   --trigger-subsync              Run subtitle sync
   --toggle-secondary-sub         Cycle secondary subtitle mode
+  --mark-watched                 Mark current video watched and advance playlist
   --toggle-subtitle-sidebar      Toggle subtitle sidebar panel
   --open-runtime-options         Open runtime options palette
   --open-session-help            Open session help modal