fix: exclude .laputa-cache.json and settings.json from vault git tracking

These files are machine-specific and should never be committed or cause
conflicts when syncing vaults across devices.

Changes:
- init_repo() now writes .gitignore with .laputa-cache.json and
  .laputa/settings.json excluded before the first commit
- .DS_Store and common editor artifacts also excluded
- openConflictFileRef falls back to openLocalFile() for non-note files
  so 'Open in editor' works for .json conflict files (opens in system
  default app, e.g. TextEdit/VS Code)
- Removed stale git.rs (replaced by git/mod.rs from refactor)
- 2 new Rust tests for .gitignore creation behavior

src/utils/url.ts

@@ -22,3 +22,11 @@ export async function openExternalUrl(url: string): Promise<void> {
     window.open(url, '_blank')
   }
 }
+
+/** Open a local file path with the system default app (e.g. TextEdit for .json). */
+export async function openLocalFile(absolutePath: string): Promise<void> {
+  if (isTauri()) {
+    const { openPath } = await import('@tauri-apps/plugin-opener')
+    await openPath(absolutePath)
+  }
+}