feat(ui): unify usage colors, fix per-bar coloring, round panel corners

Phase 0 of UI/UX polish pass. Surgical changes, no substrate migration yet.

- Extract bar_fill_color + accent_color_for into new src/usage_color.rs so
  the bubble, panel, and tray badge agree on a single 4-band usage ramp.
- Panel: color each bar from its own percent (was using max(5h, 7d) for
  both rows, so a healthy 5h bar turned red whenever 7d was full).
- Light-mode amber #B47A20 (was #E0A040, failed WCAG AA at 2.4:1).
- Codex identity: switch from white/charcoal to OpenAI teal #10A37F
  across bubble, panel stripe, and tray sweep so the surfaces share one
  brand color and the tray badge stops reading as "loading spinner".
- Panel: drop WS_BORDER, add DwmSetWindowAttribute(DWMWCP_ROUND) for
  Win11 rounded corners. Idempotent re-apply on every show() so the
  attribute survives any future destroy/recreate path. Silently no-ops
  on Win10.

src/main.rs

@@ -13,6 +13,7 @@ mod os;
 mod tray;
 mod update;
 mod usage;
+mod usage_color;
 
 // Application surface.
 mod app;