feat: threshold balloons + dark/light auto-follow

- Threshold balloons fire the cycle utilization crosses 80% or 95%
  on either provider, with the title showing "{Provider} · {N}%" and
  body translated per shipped locale. Reuses the existing
  BALLOON_COOLDOWN so notifications stay calm.
- Dark/light auto-follow: bubble's WM_SETTINGCHANGE handler now calls
  app::recheck_theme(), which re-reads HKCU\…\Personalize, updates
  state.is_dark if changed, and triggers a UI repaint + tray refresh.
  Windows posts WM_SETTINGCHANGE to every top-level window when the
  user toggles light/dark in Settings, so the bubble repaints in
  near-real-time.
- Adds 2 new i18n keys (threshold_80_body, threshold_95_body) across
  all eight shipped locales.

Bumps version to 0.1.5.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "claude-code-usage-bubble"
-version = "0.1.4"
+version = "0.1.5"
 edition = "2021"
 license = "Apache-2.0"
 description = "Floating bubble showing Claude Code and Codex usage on Windows"