[FEATURE]: Color context usage and cost by thresholds (sidebar and prompt footer) #53263

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

The session sidebar's Context block and the usage line in the prompt footer draw tokens, percent used and cost in the same muted color, whether the context is 5% or 95% full and whether the session has cost $0.10 or $100. A session about to hit its context limit, or one past the budget you had in mind, looks the same as a fresh one.

Proposal: a usage struct in tui.json, all keys optional.

• usage.context_color: "plain" (default, today's rendering) or "colored". With "colored", the token count and percent used share one color, picked from the percent used.
• usage.context_thresholds: percent-used thresholds, default [60, 70, 80, 90].
  • Up to 60% stays muted.
  • Above the first threshold it takes the theme's warning color.
  • Above the last threshold it takes the theme's error color.
  • The thresholds in between blend from warning to error.
• usage.cost_thresholds: dollar thresholds for the session cost, using the same rule. Unset by default, so cost stays muted. [5] means "warn above $5", and [5, 20] means warning above $5 and error above $20.

Colors come only from the active theme, so every theme gets a matching ramp. The settings apply to:

• the sidebar's expanded block;
• every width variant of the compact line from [FEATURE]: Compact one-line display for the sidebar Context block #53200;
• the prompt footer's 95.0K (95%) · $3.70.