feat(blame): add file blame dialog with git porcelain parsing

This change introduces a new backend command to fetch file blame using
git's line-porcelain output and returns structured per-line metadata.
The UI adds a BlameDialog that groups lines by commit, highlights
uncommitted changes, and styles the dialog to match the updated theme.

- Add get_file_blame command and parsing with uncommitted detection
- Wire BlameDialog into the explorer file node actions
- Add blame UI component and supporting types and styles

src/lib/components/RepoLoadingOverlay.svelte

@@ -47,8 +47,7 @@
     z-index: 400;
     display: grid;
     place-items: center;
-    background: rgba(6, 6, 14, 0.72);
-    backdrop-filter: blur(8px);
+    background: #050712;
     animation: overlay-in 180ms ease;
   }