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/git.ts

@@ -5,6 +5,7 @@ import type {
   CommitAiProvider,
   CommitAiStatus,
   ConflictFile,
+  GitBlameResult,
   GitBranch,
   GitCommit,
   GitCommitComparison,
@@ -322,6 +323,10 @@ export function cancelFileHistory(requestId: string): Promise<void> {
   return invoke<void>("cancel_file_history", { requestId });
 }
 
+export function getFileBlame(path: string, file: string): Promise<GitBlameResult> {
+  return invoke<GitBlameResult>("get_file_blame", { path, file });
+}
+
 export function compareCommits(
   path: string,
   from: string,