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>

samples/example.diff

@@ -0,0 +1,35 @@
+diff --git a/src/greet.ts b/src/greet.ts
+index 1a2b3c4..5d6e7f8 100644
+--- a/src/greet.ts
++++ b/src/greet.ts
+@@ -1,8 +1,9 @@
+ // Greeting helpers
+-export function greet(name) {
+-  return "Hello " + name;
++export function greet(name: string): string {
++  const trimmed = name.trim();
++  return `Hello, ${trimmed || "world"}!`;
+ }
+
+-console.log(greet("there"));
++console.log(greet("  there  "));
+diff --git a/scripts/run.py b/scripts/run.py
+index aaaaaaa..bbbbbbb 100644
+--- a/scripts/run.py
++++ b/scripts/run.py
+@@ -1,4 +1,5 @@
+ import sys
+
+-def main():
+-    print("hi")
++def main() -> int:
++    print("hello from python")
++    return 0
+diff --git a/README.md b/README.md
+new file mode 100644
+index 0000000..ccccccc
+--- /dev/null
++++ b/README.md
+@@ -0,0 +1,2 @@
++# Demo
++A tiny sample used to exercise the highlighter.