feat: initial port of claude-code-usage-monitor as a floating bubble

Windows-only floating, draggable circular bubble showing Claude Code
and Codex usage. Derivative of CodeZeno/Claude-Code-Usage-Monitor (MIT),
relicensed under Apache 2.0 with upstream attribution in NOTICE.

- Ported verbatim: poller, updater, tray_icon, theme, localization,
  diagnose, models (~2,700 LOC)
- Original: bubble (circular layered window, drag-anywhere via
  WM_NCHITTEST+HTCAPTION, snap-to-edge, Ctrl+Wheel resize, auto-hide
  on fullscreen), panel (expanded 5h/7d view), app (orchestrator,
  single-instance mutex, polling thread, context menu, dual-bubble
  lifecycle), settings (settings.json persistence)
- Cargo.toml features cover Win32 GDI, HiDpi, Registry, Threading,
  Shell, WindowsAndMessaging, and KeyboardAndMouse

src/icons/48.svg

@@ -0,0 +1,14 @@
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+  <rect x="28" y="10" width="6" height="12" rx="2" fill="#F3D6CC"></rect>
+  <path d="M 30 10 L 31 10 L 31 22 L 30 22 C 28.895 22 28 21.105 28 20 L 28 12 C 28 10.895 28.895 10 30 10 Z" fill="#D97757"></path>
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+  <rect x="35" y="26" width="6" height="12" rx="2" fill="#F3D6CC"></rect>
+</svg>