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>

README.md

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+# Diff Highlighter
+
+A Chrome (Manifest V3) extension that highlights `.diff` and `.patch` files in the
+browser: **additions green, deletions red**, with hunk/file headers styled and the
+underlying code **syntax-highlighted** by language. Works on GitHub/Bitbucket PR
+`.diff`/`.patch` URLs, raw files, and local `file://` diffs. Light/dark aware, fast,
+and fully toggleable.
+
+## How it works
+
+- A content script runs only on `text/plain` pages whose URL ends in `.diff`/`.patch`
+  (the `include_globs` matcher covers query strings too, e.g. `…/123.diff?token=…`).
+- It reads the browser-rendered `<pre>`, parses the unified-diff structure, and:
+  1. **Instantly** repaints every line with its diff color (green/red/context/headers).
+  2. **Progressively** layers syntax highlighting via [highlight.js](https://highlightjs.org/),
+     processed in idle-time chunks so large diffs stay responsive. Above ~50k highlightable
+     lines, syntax is skipped to preserve speed (diff colors remain).
+- Toggling is instant and reload-free: the popup writes `chrome.storage.sync`, and the
+  content script re-renders on change. `Alt+D` flips the master toggle from anywhere.
+- Only the `storage` permission is requested — declarative content scripts need no host
+  permissions.
+
+## Toggles
+
+| Control | Effect |
+| --- | --- |
+| **Highlighting** (popup) | Master switch for the green/red diff layer. |
+| **Syntax highlighting** (popup) | Layer language-aware token colors on top. |
+| `Alt+D` | Flip the master switch (rebind at `chrome://extensions/shortcuts`). |
+
+## Build
+
+Requires Node 20+.
+
+```sh
+npm ci
+npm run build      # type-checks, then bundles to dist/
+npm run watch      # rebuild on change (dev)
+```
+
+`dist/` is the unpacked extension (git-ignored).
+
+## Load in Chrome
+
+1. `chrome://extensions` → enable **Developer mode**.
+2. **Load unpacked** → select the `dist/` folder.
+3. For local diffs, open the extension's **Details** page and enable
+   **“Allow access to file URLs.”** Then try `samples/example.diff`.
+
+## Continuous integration (Codeberg / Forgejo Actions)
+
+CI lives in [`.forgejo/workflows/ci.yml`](.forgejo/workflows/ci.yml) and runs
+`npm ci → typecheck → build` in a `node:22-bookworm` container — the same steps as a
+local build, so a green local build means green CI.
+
+To wire it up on Codeberg:
+
+1. Push this repo to Codeberg:
+   ```sh
+   git remote add origin https://codeberg.org/<you>/diff-highlighter.git
+   git push -u origin main
+   ```
+2. Connect a runner: repo **Settings → Actions → Runners** (or use Codeberg's hosted
+   runners). If using a hosted runner, change `runs-on: docker` to its label
+   (e.g. `codeberg-tiny`).
+
+## Scope
+
+Triggers on URLs/files whose path ends in `.diff`/`.patch` and that render as plain text.
+In-app **HTML** diff views (GitHub's "Files changed", Bitbucket's `/diff` endpoint) are out
+of scope — those are full HTML pages, not raw text.
+
+## Layout
+
+```
+src/
+  manifest.json      MV3 manifest
+  content.ts         detect + render + live toggle
+  diff-parser.ts     pure diff parsing / language detection
+  background.ts      defaults + Alt+D command
+  popup/             popup UI
+  styles/            diff.css, hljs-theme.css, popup.css (all light/dark)
+scripts/             build.mjs (esbuild), make-icons.mjs
+.forgejo/workflows/  CI
+```