[Feat] Display model context window badge in model selector

[samzong]

What's changed?

• Add formatContextWindow() helper to render token counts as human-readable labels (200K, 1M)
• Display context window as a blue info badge next to the model name in both the trigger button and dropdown menu items
• Existing reasoning "R" badge (green accent) remains unchanged

Why

• Model selector only showed model name and reasoning badge — context window size is useful information for model selection

[github-actions]

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