fix: rename Wisp theme to Custom and hide accent picker for presets

[barrydeen]

Summary

• Rename the "Wisp" theme to "Custom" across Themes.kt, Theme.kt, InterfacePreferences.kt, and InterfaceScreen.kt
• Wrap the accent color picker section in if (isCustomTheme) so it only appears when the Custom theme is selected
• Preset themes (Nord, Dracula, etc.) no longer show the irrelevant color picker

Test plan

• Settings → Interface → verify "Custom" label on theme card
• Select a preset theme (e.g. Nord) → accent color section disappears
• Select "Custom" → accent color section reappears with correct color
• Changing accent color still updates the theme live