fix(settings): persist font size changes in Appearance theme editor

[shouwang0527]

Fixes #1381.

Summary

Two-file fix for the font size bug:

1. ThemeEngine.saveUserTheme now upserts the saved theme into availableThemes synchronously before kicking off reloadAvailableThemes(). The previous code relied on the async reload completing before the SwiftUI binding setter ran, so the new (Custom) theme was missing from the list on the same tick and the picker silently reverted to the original built-in.
2. ThemeEditorFontsSection.updateFont no longer calls engine.activateTheme(copy) directly. Activation goes through the onThemeDuplicated callback, which also writes preferredLightThemeId / preferredDarkThemeId. Without that, relaunch reconciliation reverted to the built-in theme and the font size change was lost.

Together these fix all three symptoms in the issue: change applies immediately, persists across relaunch, and no orphan (Custom) themes accumulate per adjustment.

Test plan

• Open Settings > Appearance > Fonts, change Editor Font Size from 13 to 18, confirm SQL editor updates immediately.
• Quit and relaunch, confirm size is still 18 and the active theme is the saved (Custom) theme.
• Repeat the size change and confirm only one (Custom) theme exists in ~/Library/Application Support/TablePro/Themes/.

Local build skipped (no Xcode locally), relying on CI.

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[datlechin]

LGTM, thank you.

[shouwang0527]

Thanks @datlechin for the quick review and merge!

Since this fixes a regression that makes the editor font feel too small and any adjustment in Appearance > Fonts gets silently reverted, it's a noticeable papercut for daily use. Would it be possible to include this in the next release? Happy to help verify a pre-release build if that's useful.

Thanks again for maintaining TablePro.