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/BranchPanel.svelte

@@ -50,6 +50,13 @@
     createOpen = false;
     localOpen = true;
   }
+
+  function checkoutOnDoubleClick(event: MouseEvent, branch: GitBranchInfo) {
+    if (branch.current || isBusy) return;
+    const target = event.target instanceof HTMLElement ? event.target : null;
+    if (target?.closest("button")) return;
+    onCheckout(branch);
+  }
 </script>
 
 <section class="panel grid grid-rows-[auto_1fr] overflow-hidden" aria-label="Branches">
@@ -121,7 +128,12 @@
             <div class="branch-empty">No local branches.</div>
           {:else}
             {#each localBranches as branch (branch.name)}
-              <article class="branch-row" class:current={branch.current}>
+              <article
+                class="branch-row"
+                class:current={branch.current}
+                ondblclick={(event) => checkoutOnDoubleClick(event, branch)}
+                title={branch.current ? "Current branch" : "Double-click to checkout"}
+              >
                 <div class="branch-info">
                   <GitBranch size={16} aria-hidden="true" />
                   <div>
@@ -169,7 +181,12 @@
             <div class="branch-empty">No remote branches.</div>
           {:else}
             {#each remoteBranches as branch (branch.name)}
-              <article class="branch-row" class:current={branch.current}>
+              <article
+                class="branch-row"
+                class:current={branch.current}
+                ondblclick={(event) => checkoutOnDoubleClick(event, branch)}
+                title={branch.current ? "Current branch" : "Double-click to checkout"}
+              >
                 <div class="branch-info">
                   <GitBranch size={16} aria-hidden="true" />
                   <div>