Initial commit: Diff Highlighter Chrome extension (MV3, TypeScript)

Highlights .diff/.patch files inline — additions green, deletions red,
hunk/file headers styled, with toggleable highlight.js syntax highlighting.

- MV3, content script gated on text/plain + URL ending in .diff/.patch
- Two-phase render: instant diff colors, then chunked idle-time syntax
- Popup switches + Alt+D shortcut, persisted in chrome.storage.sync, live re-render
- Light/dark via prefers-color-scheme; only the `storage` permission
- TypeScript + esbuild build; Forgejo Actions CI (typecheck + build)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/settings.ts

@@ -0,0 +1,12 @@
+/** User-facing toggles, persisted in chrome.storage.sync. */
+export interface Settings {
+  /** Master switch for the green/red diff layer. */
+  enabled: boolean;
+  /** Layer language-aware syntax highlighting on top of the diff colors. */
+  syntax: boolean;
+}
+
+export const DEFAULT_SETTINGS: Settings = {
+  enabled: true,
+  syntax: true,
+};