Context

PR #1684 added enable_extended_context support for the Claude 1M context window. When this mode is active, the TUI has no visual indicator — users cannot tell from the dashboard that they are in extended context mode and subject to long-context pricing.

Task

Add a visible status indicator in the TUI when enable_extended_context = true:

• Status bar item: 1M CTX or similar badge next to the model name
• Optionally: show a cost indicator (e.g., $$ suffix)
• Per TUI Rules: any implicit mode must have a visible indicator so users always know what is happening

Notes

• Read context_window() from the active provider to determine if 1M is in effect
• Cosmetic but important for cost transparency

Follows up on #1649.