Implement interactive hunk staging and discarding

Introduce a new dialog that allows users to view file changes as individual hunks and selectively stage, unstage, or discard them. This provides more granular control over modifications, similar to `git add -p`.

Enhance branch panel usability by allowing double-click to checkout a branch.

src/lib/types.ts

@@ -24,6 +24,8 @@ export interface GitFileStatus {
   unstaged: FileStatusKind | null;
 }
 
+export type PatchApplyAction = "stage" | "unstage" | "discard-unstaged" | "discard-staged";
+
 export interface GitBranch {
   name: string;
   current: boolean;