Fix Monaco theme handling for DiffTab consistency

[Copilot]

DiffTab wasn't applying themes correctly when switching tabs, while CodeEditor worked fine. Root cause: theme names mismatched - components used theme="pyxis-custom" but monaco-themes.ts defined pyxis-dark/pyxis-light.

Changes

• Unified theme name: Define single pyxis-custom theme matching component props
• Simplified light/dark detection: Use LIGHT_THEMES list with themeName instead of computing from colors
• Cache by themeName: Only re-define theme when themeName changes

// Before: color-based detection with separate theme names
const useLight = isHexLight(bg) || /white|fff/i.test(background);
const targetTheme = useLight ? 'pyxis-light' : 'pyxis-dark';

// After: simple themeName check
const LIGHT_THEMES = ['light', 'solarizedLight', 'pastelSoft'];
const useLight = LIGHT_THEMES.includes(themeName);
mon.editor.defineTheme('pyxis-custom', { base: useLight ? 'vs' : 'vs-dark', ... });

Updated callers (MonacoEditor, DiffTab, AIReviewTab) to pass themeName from useTheme().

Original prompt

diffタブで、確かにテーマ再設定毎回するようにするとちゃんとテーマ再設定される。しかし、テーマをキャッシュしておくようにすると、うまくいかんかった。なぜ、codeeditor同士のタブ切り替えではthemeがちゃんとグローバルで、再設定しなくてよくて、diffだと今の実装にならんとバグる？慎重に調査して、修正して、毎回テーマ設定は知って重くならないようにしたい。

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.