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-tauri/src/git.rs

@@ -1,6 +1,7 @@
 use serde::{Deserialize, Serialize};
 use std::process::Command;
 
+#[cfg(target_os = "windows")]
 use crate::process_ext::HideWindow;
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -37,9 +38,38 @@ pub struct GitLogEntry {
     pub message: String,
 }
 
+/// Builds the WSL argument list for running a git command at a Linux path.
+/// Extracted for testability without requiring WSL to be available.
+#[cfg(any(target_os = "windows", test))]
+fn build_wsl_git_args<'a>(working_dir: &'a str, args: &[&'a str]) -> Vec<&'a str> {
+    let mut wsl_args = vec!["--", "git", "-C", working_dir];
+    wsl_args.extend_from_slice(args);
+    wsl_args
+}
+
 fn run_git_command(working_dir: &str, args: &[&str]) -> Result<String, String> {
+    #[cfg(target_os = "windows")]
+    let output = {
+        if working_dir.starts_with('/') {
+            // WSL/Linux path — run git through WSL so it can resolve the path correctly.
+            let wsl_args = build_wsl_git_args(working_dir, args);
+            Command::new("wsl")
+                .hide_window()
+                .args(&wsl_args)
+                .output()
+                .map_err(|e| format!("Failed to execute git via WSL: {}", e))?
+        } else {
+            Command::new("git")
+                .hide_window()
+                .args(args)
+                .current_dir(working_dir)
+                .output()
+                .map_err(|e| format!("Failed to execute git: {}", e))?
+        }
+    };
+
+    #[cfg(not(target_os = "windows"))]
     let output = Command::new("git")
-        .hide_window()
         .args(args)
         .current_dir(working_dir)
         .output()
@@ -297,6 +327,26 @@ mod tests {
     use std::io::Write;
     use tempfile::TempDir;
 
+    // ==================== build_wsl_git_args tests ====================
+
+    #[test]
+    fn test_build_wsl_git_args_structure() {
+        let args = build_wsl_git_args("/home/naomi/code/project", &["status", "--porcelain=v1"]);
+        assert_eq!(args[0], "--");
+        assert_eq!(args[1], "git");
+        assert_eq!(args[2], "-C");
+        assert_eq!(args[3], "/home/naomi/code/project");
+        assert_eq!(args[4], "status");
+        assert_eq!(args[5], "--porcelain=v1");
+        assert_eq!(args.len(), 6);
+    }
+
+    #[test]
+    fn test_build_wsl_git_args_no_extra_args() {
+        let args = build_wsl_git_args("/home/user/repo", &["init"]);
+        assert_eq!(args, vec!["--", "git", "-C", "/home/user/repo", "init"]);
+    }
+
     // Helper to create a git repository in a temp directory
     fn create_test_repo() -> TempDir {
         let temp_dir = TempDir::new().unwrap();