feat(overlay): add in-app changelog modal

- Add a changelog modal opened from tray "View Changelog" or the update notification's "What's New" button; renders in-player or in its own window like the help modal
- Fetch changelog from the newest published release so newer-than-installed notes are visible, falling back to the bundled CHANGELOG.md on failure
- Group versions by minor line (current expanded, older folded), badge the installed version, tag newer ones "New"
- Support J/K/arrow navigation, Enter to fold/unfold, R to refetch, Esc to close
- Add changelog parsing/semver-compare utils and changelog IPC channel/runtime; bundle CHANGELOG.md into packaged builds
- Replace release/release-notes.md with changes/changelog-modal.md changeset entry

docs-site/architecture.md

@@ -75,8 +75,8 @@ src/
   renderer/                # Overlay renderer (modularized UI/runtime)
     handlers/              # Keyboard/mouse/gamepad interaction modules
     modals/                # Modal flows (Jimaku, Kiku, subsync, runtime options, session help,
-                           #   character dictionary, playlist browser, subtitle sidebar,
-                           #   YouTube track picker, controller config/debug/select)
+                           #   changelog, character dictionary, playlist browser, subtitle
+                           #   sidebar, YouTube track picker, controller config/debug/select)
     positioning/           # Subtitle position controller (drag-to-reposition)
   settings/                # Settings window UI (model, controls, markup)
   types/                   # Domain type modules (anki, config, integrations, ...)