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/components/StatusPanel.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-  import { Check, RotateCcw, Undo2 } from "@lucide/svelte";
+  import { Check, FileDiff, RotateCcw, Undo2 } from "@lucide/svelte";
   import type { FileStatusKind, GitFileStatus, GitStatus } from "../types";
 
   interface Props {
@@ -12,6 +12,7 @@
     onStage: (file: GitFileStatus) => void;
     onUnstage: (file: GitFileStatus) => void;
     onDiscard: (file: GitFileStatus, staged: boolean) => void;
+    onPatch: (file: GitFileStatus, staged: boolean) => void;
     onStageAll: () => void;
     onUnstageAll: () => void;
   }
@@ -26,6 +27,7 @@
     onStage = () => {},
     onUnstage = () => {},
     onDiscard = () => {},
+    onPatch = () => {},
     onStageAll = () => {},
     onUnstageAll = () => {},
   }: Props = $props();
@@ -46,6 +48,10 @@
     return file.old_path ? `${baseName(file.old_path)} -> ${baseName(file.path)}` : baseName(file.path);
   }
 
+  function canPatch(kind: FileStatusKind | null): boolean {
+    return kind === "modified";
+  }
+
   let hasUnstaged = $derived(changedFiles.some((f) => f.unstaged !== null));
   let hasStaged = $derived(changedFiles.some((f) => f.staged !== null));
 </script>
@@ -113,6 +119,10 @@
                     <Undo2 size={14} aria-hidden="true" />
                     Unstage
                   </button>
+                  <button class="btn-sm" type="button" onclick={() => onPatch(file, true)} disabled={isBusy || !canPatch(file.staged)} title="Stage, unstage, or discard selected lines">
+                    <FileDiff size={14} aria-hidden="true" />
+                    Lines
+                  </button>
                   <button class="btn-sm" type="button" onclick={() => onDiscard(file, true)} disabled={isBusy} title="Discard staged changes">
                     <RotateCcw size={14} aria-hidden="true" />
                     Discard
@@ -134,6 +144,10 @@
                     <Check size={14} aria-hidden="true" />
                     Stage
                   </button>
+                  <button class="btn-sm" type="button" onclick={() => onPatch(file, false)} disabled={isBusy || !canPatch(file.unstaged)} title="Stage or discard selected lines">
+                    <FileDiff size={14} aria-hidden="true" />
+                    Lines
+                  </button>
                   <button class="btn-sm" type="button" onclick={() => onDiscard(file, false)} disabled={isBusy} title="Discard unstaged changes">
                     <RotateCcw size={14} aria-hidden="true" />
                     Discard