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/utils/diffParser.ts

@@ -0,0 +1,111 @@
+export type DiffLineType =
+  | "file-header"
+  | "hunk-header"
+  | "added"
+  | "removed"
+  | "context"
+  | "no-newline";
+
+export interface ParsedDiffLine {
+  type: DiffLineType;
+  content: string;
+  oldLineNumber: number | null;
+  newLineNumber: number | null;
+}
+
+const FILE_HEADER_PREFIXES = [
+  "diff ",
+  "index ",
+  "--- ",
+  "+++ ",
+  "new file",
+  "deleted file",
+  "old mode",
+  "new mode",
+  "rename ",
+  "similarity ",
+];
+
+export function parseDiff(diffContent: string): ParsedDiffLine[] {
+  const result: ParsedDiffLine[] = [];
+  let oldLine = 0;
+  let newLine = 0;
+
+  for (const line of diffContent.split("\n")) {
+    if (FILE_HEADER_PREFIXES.some((prefix) => line.startsWith(prefix))) {
+      result.push({ type: "file-header", content: line, oldLineNumber: null, newLineNumber: null });
+    } else if (line.startsWith("@@")) {
+      const match = line.match(/@@ -(\d+)(?:,\d+)? \+(\d+)(?:,\d+)? @@/);
+      if (match) {
+        oldLine = parseInt(match[1], 10);
+        newLine = parseInt(match[2], 10);
+      }
+      result.push({ type: "hunk-header", content: line, oldLineNumber: null, newLineNumber: null });
+    } else if (line.startsWith("+")) {
+      result.push({
+        type: "added",
+        content: line.slice(1),
+        oldLineNumber: null,
+        newLineNumber: newLine++,
+      });
+    } else if (line.startsWith("-")) {
+      result.push({
+        type: "removed",
+        content: line.slice(1),
+        oldLineNumber: oldLine++,
+        newLineNumber: null,
+      });
+    } else if (line.startsWith(" ")) {
+      result.push({
+        type: "context",
+        content: line.slice(1),
+        oldLineNumber: oldLine++,
+        newLineNumber: newLine++,
+      });
+    } else if (line === "\\ No newline at end of file") {
+      result.push({ type: "no-newline", content: line, oldLineNumber: null, newLineNumber: null });
+    }
+    // Skip empty trailing lines
+  }
+
+  return result;
+}
+
+const EXTENSION_MAP: Record<string, string> = {
+  ts: "typescript",
+  tsx: "typescript",
+  js: "javascript",
+  jsx: "javascript",
+  rs: "rust",
+  py: "python",
+  svelte: "xml",
+  css: "css",
+  scss: "scss",
+  less: "less",
+  html: "html",
+  json: "json",
+  md: "markdown",
+  toml: "ini",
+  yaml: "yaml",
+  yml: "yaml",
+  sh: "bash",
+  bash: "bash",
+  go: "go",
+  java: "java",
+  cpp: "cpp",
+  c: "c",
+  rb: "ruby",
+  php: "php",
+  sql: "sql",
+  kt: "kotlin",
+  swift: "swift",
+  cs: "csharp",
+  r: "r",
+  lua: "lua",
+  xml: "xml",
+};
+
+export function detectLanguage(filePath: string): string {
+  const ext = filePath.split(".").pop()?.toLowerCase() ?? "";
+  return EXTENSION_MAP[ext] ?? "plaintext";
+}