feat: Add project file tree viewer to Git page

Add Files tab with browsable project structure and file content viewer.
New components: ProjectTree for navigation, FileViewer for content display.
Backend endpoints: /api/git/tree and /api/git/file.

frontend/src/types/git.ts

@@ -68,3 +68,18 @@ export interface CompareResult {
     deletions: number
   }
 }
+
+export interface TreeNode {
+  name: string
+  path: string
+  type: 'file' | 'directory'
+  children?: TreeNode[]
+}
+
+export interface FileContent {
+  path: string
+  isBinary: boolean
+  content: string | null
+  size: number
+  extension?: string
+}