[Feature]: Allow custom terminal font fallback stacks

## Problem or use case

Orca's terminal supports a primary font and built-in fallbacks, but users cannot choose the fallback fonts or their order.

This limits Orca's multilingual experience. Most programming fonts do not cover every writing system, so Chinese, Japanese, Korean, Arabic, Thai, Devanagari, and other text may be rendered with inconsistent platform-selected fonts.

Configurable fallback fonts would make multilingual terminal content more readable and predictable, helping Orca better serve users across languages and regions.

## Proposed solution

Allow users to configure an ordered list of terminal fallback fonts.

The effective order should be:

`Primary font → user-defined fallbacks → Orca's existing defaults`

When the setting is empty or a font is unavailable, Orca should preserve its current fallback behavior.

## Alternatives or additional context

For example, a user could select `MesloLGS NF` as the primary font and `Microsoft YaHei UI` as a fallback for missing Chinese glyphs. This is a general multilingual feature, not a Chinese-specific workaround.