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/ExplorerPanel.svelte

@@ -18,6 +18,7 @@
     Folder,
     FolderOpen,
     ExternalLink,
+    History,
     Terminal,
   } from "@lucide/svelte";
   import { languageIconForPath } from "../languageIcons";
@@ -36,6 +37,7 @@
     onCollapseAllFolders: () => void;
     onSelectNode: (node: ExplorerNode) => void;
     onOpenFile: (node: ExplorerNode) => void;
+    onBlame: (node: ExplorerNode) => void;
   }
 
   let {
@@ -50,6 +52,7 @@
     onCollapseAllFolders = () => {},
     onSelectNode = () => {},
     onOpenFile = () => {},
+    onBlame = () => {},
   }: Props = $props();
 
   let contextNode = $state<ExplorerNode | null>(null);
@@ -182,6 +185,13 @@
     onOpenFile(node);
   }
 
+  function openContextBlame() {
+    const node = contextNode;
+    if (!node || node.kind !== "file") return;
+    closeFileContextMenu();
+    onBlame(node);
+  }
+
   function handleWindowKeydown(event: KeyboardEvent) {
     if (event.key === "Escape") closeFileContextMenu();
   }
@@ -335,5 +345,15 @@
       <ExternalLink size={14} aria-hidden="true" />
       Open in Explorer
     </button>
+    <button
+      type="button"
+      role="menuitem"
+      onclick={openContextBlame}
+      disabled={!contextNode.tracked || contextNode.status === "deleted"}
+      title={!contextNode.tracked || contextNode.status === "deleted" ? "Blame is only available for tracked files" : "Show blame for this file"}
+    >
+      <History size={14} aria-hidden="true" />
+      Blame
+    </button>
   </div>
 {/if}