feat: fix git window and add pretty diff viewer (#178)

## Summary

- **Fix git window "Not a git repository" error** — The working directory received from Claude Code is a WSL Linux path (e.g. `/home/naomi/...`), but git commands were being run as native Windows processes with `.current_dir()`. Windows can't resolve WSL paths, causing `git rev-parse --git-dir` to fail. Fixed by routing git commands through `wsl -- git -C <path>` when the working directory starts with `/`.

- **Add syntax highlighting and line numbers to diff view** — Replaced the raw `<pre>` block with a proper `DiffViewer` component featuring:
  - Old/new line number columns with correct tracking across hunks
  - Colour-coded gutter (`+`/`-`) with green/red row backgrounds
  - Syntax highlighting via `highlight.js` using the detected file language, respecting all app themes via `--hljs-*` CSS variables
  - Styled hunk headers and file headers

## New files

- `src/lib/utils/diffParser.ts` — pure diff parsing logic
- `src/lib/utils/diffParser.test.ts` — 30 tests covering all line types, line number tracking, and language detection
- `src/lib/components/DiffViewer.svelte` — the pretty diff viewer component

✨ This pull request was created with help from Hikari~ 🌸

Reviewed-on: #178
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

src/lib/components/GitPanel.svelte

@@ -2,6 +2,7 @@
   import { invoke } from "@tauri-apps/api/core";
   import { onMount, onDestroy } from "svelte";
   import { claudeStore } from "$lib/stores/claude";
+  import DiffViewer from "$lib/components/DiffViewer.svelte";
 
   interface GitFileChange {
     path: string;
@@ -600,7 +601,9 @@
           <h3>📄 {diffFile}</h3>
           <button on:click={() => (showDiff = false)} title="Close">✕</button>
         </div>
-        <pre class="diff-content">{diffContent || "(No changes)"}</pre>
+        <div class="diff-content">
+          <DiffViewer {diffContent} filePath={diffFile ?? ""} />
+        </div>
       </div>
     </div>
   {/if}
@@ -1096,12 +1099,7 @@
   .diff-content {
     flex: 1;
     overflow: auto;
-    padding: 1rem;
     margin: 0;
-    font-family: var(--font-mono);
-    font-size: 0.85rem;
-    line-height: 1.4;
-    white-space: pre;
     background: var(--bg-primary);
   }
 </style>