[Windows Desktop] Restore UI font size 18+ and do not silently reset existing values

Codex version

• Windows Store/MSIX package: 26.818.2872.0 x64
• Desktop app: 26.818.22352
• Bundled agent/app-server: 0.148.0-alpha.21

Problem

After updating Codex Desktop, a previously valid UI font setting of sansFontSize = 18 was silently rejected. The current packaged settings schema accepts only 11 through 16, with a default of 14.

At startup, Desktop logged:

[settings-store] Dropping invalid desktop setting key=sansFontSize

The result was an immediate and noticeable reduction from 18 to the default 14. The Settings UI can restore only 16, which is still too small for comfortable extended use on this monitor.

This is both an accessibility regression and a settings-migration problem: an application update reduced the valid range, discarded an existing preference, and changed the UI without warning.

Reproduction

1. Start with an earlier Codex Desktop profile containing desktop.sansFontSize = 18.
2. Update to the current Windows Store build.
3. Launch Codex Desktop.
4. Observe that the interface is substantially smaller.
5. Observe the settings-store warning above in the Desktop log.
6. Open the UI font-size setting and observe that 16 is now the maximum.

Expected behavior

• Support UI font size 18 again, preferably with additional larger accessibility options.
• Preserve settings that were valid in earlier releases.
• If a range must contract, clamp 18 to 16 instead of silently dropping it to the default 14.
• Notify the user when an update migrates or rejects a visual/accessibility preference.
• Keep the selected value persistent across updates and restarts.

Additional context

This independently confirms the font migration reported in #39713, but is intentionally scoped to restoring the UI font range and preventing silent accessibility-setting resets. That report also covers a separate focus-dependent app-zoom problem.

OpenAI in-app feedback reference: 019e2da7-2087-7643-8c18-ae6db12a35b9

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