[BUG] Context window progress bar fill width doesn't match displayed percentage (under-fills by ~15-20%)

Description

In the usage / context overlay panel, the Context window progress bar is visually filled significantly less than the percentage shown in the label. The numeric label reads 82%, but the blue filled portion of the bar reaches only ~64% of the track width — an under-fill of roughly 15-20%.

The "Plan usage" bars below it (5-hour limit 17%, Weekly 4%) appear to render at roughly the correct width, so the discrepancy seems specific to the context-window bar.

<img width="412" height="266" alt="Image" src="https://github.com/user-attachments/assets/b2660a5c-c466-4ff6-b6c8-d0a0c5260070" />

Steps to reproduce

1. Open the usage overlay panel (the floating panel that shows "Context window … (xx%)" and "Plan usage").
2. Let the context window fill up (here: 164.6k / 200.0k = 82%).
3. Compare the width of the blue fill against the label percentage.

Expected

The fill width should match the displayed percentage — at 82% the bar should be filled ~82% of the track.

Actual

• Label: 164.6k / 200.0k (82%)
• Bar fill measured from the screenshot: ~64% of the track width
• Mismatch: ~15-18% under-filled

Possible cause: the bar may be scaling against a larger denominator than 200k (e.g. reserving headroom for an auto-compact window) while the label divides by 200k. Or a max-width / inset rounding bug in the fill calculation.

Environment

• OS: macOS (Darwin 25.3.0)
• Model shown: Opus 4.8
• Surface: usage/context overlay panel

A screenshot is available and can be attached to this issue.