feat(usage): budget-aware coloring for the Extra Usage widgets

[zachthedev]

What

Implements #467: optional budget-aware coloring for the three Extra Usage widgets
(extra-usage-used, extra-usage-remaining, extra-usage-utilization), so the value visibly
shifts color as you approach your monthly spend limit instead of sitting in one static color.

Design

• New optional Widget hook getDynamicColor(item, context): string | undefined (src/types/Widget.ts).
  A widget can override its own foreground color from live render data.
• The renderer prefers it over item.color ?? defaultColor at both per-widget color sites (the normal
  path and the powerline path). The global overrideForegroundColor still takes precedence, so existing
  precedence is unchanged, and any widget that does not implement the hook renders exactly as before.
• The three Extra Usage widgets implement it, keyed on the API utilization percentage: green below the
  warn threshold, yellow in the warn band, red at the critical threshold and above.

Opt-in and configuration

• Off by default. Toggle per widget with the new (b)udget colors keybind in the editor (it sets a
  budgetColors metadata flag). With the flag off, the widget renders in its static configured color
  exactly as today.
• The three tier colors default to plain green / yellow / red and are each overridable per widget
  via metadata (budgetColorOk / budgetColorWarn / budgetColorCrit), accepting the same color
  formats as item.color (named, ansi256:N, hex:RRGGBB).
• Thresholds: warn at 75%, critical at 90% utilization.

Default behavior unchanged

Without the opt-in flag, every widget renders exactly as it does now: the dynamic color resolves to
undefined and the renderer falls through to the existing item.color ?? defaultColor.

Tests

• New budget-color suite: threshold escalation, custom hex overrides, the opt-in toggle, the keybind
  and editor modifier, and each widget's getDynamicColor.
• Updated the three Extra Usage keybind assertions for the new (b) bind.
• Full suite: 1597 pass, lint clean.

Notes

Happy to split into two commits for review (interface + renderer hook, then the widget implementations)
if you would prefer to take it in pieces.

Closes #467