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/App.svelte

@@ -14,6 +14,7 @@
   import FileHistoryPanel from "./lib/components/FileHistoryPanel.svelte";
   import GlobalSearchDialog from "./lib/components/GlobalSearchDialog.svelte";
   import HistoryPanel from "./lib/components/HistoryPanel.svelte";
+  import LinePatchDialog from "./lib/components/LinePatchDialog.svelte";
   import NewBranchDialog from "./lib/components/NewBranchDialog.svelte";
   import RepoLoadingOverlay from "./lib/components/RepoLoadingOverlay.svelte";
   import ResolveDialog from "./lib/components/ResolveDialog.svelte";
@@ -25,6 +26,7 @@
     commit,
     compareCommits,
     cancelCodeSearch,
+    applyFilePatch,
     createBranch,
     diffFileAgainstWorkingTree,
     compareFileToParent,
@@ -41,6 +43,7 @@
     credLoad,
     credSave,
     credDelete,
+    getFilePatch,
     readConflict,
     resolveConflict,
     resolveConflictSide,
@@ -65,6 +68,7 @@
     GitRepositoryFile,
     GitSearchHit,
     GitStatus,
+    PatchApplyAction,
     PreparedResolution,
     StoredCredential,
   } from "./lib/types";
@@ -103,6 +107,12 @@
   let selectedDiffPath = "";
   let diffHighlightQuery = "";
   let pendingRestoreFile: { commit: GitCommit; file: GitCommitFile } | null = null;
+  let linePatchOpen = false;
+  let linePatchFile: GitFileStatus | null = null;
+  let linePatchStaged = false;
+  let linePatchText = "";
+  let linePatchLoading = false;
+  let linePatchError = "";
   let globalSearchOpen = false;
   let lastSearchQuery = "";
   let globalSearchResults: GitSearchHit[] = [];
@@ -675,6 +685,77 @@
     });
   }
 
+  async function openLinePatch(file: GitFileStatus, staged: boolean) {
+    if (!activeRepoPath) return;
+    linePatchOpen = true;
+    linePatchFile = file;
+    linePatchStaged = staged;
+    linePatchText = "";
+    linePatchError = "";
+    linePatchLoading = true;
+
+    try {
+      linePatchText = await getFilePatch(activeRepoPath, file.path, staged);
+    } catch (error) {
+      linePatchError = errorToMessage(error);
+      errorMessage = linePatchError;
+    } finally {
+      linePatchLoading = false;
+    }
+  }
+
+  async function refreshLinePatch() {
+    if (!activeRepoPath || !linePatchFile) return;
+    await openLinePatch(linePatchFile, linePatchStaged);
+  }
+
+  function closeLinePatch() {
+    if (isBusy) return;
+    linePatchOpen = false;
+    linePatchFile = null;
+    linePatchText = "";
+    linePatchError = "";
+  }
+
+  function patchOperationLabel(action: PatchApplyAction, file: GitFileStatus): string {
+    switch (action) {
+      case "stage":
+        return `Staging hunk in ${file.path}`;
+      case "unstage":
+        return `Unstaging hunk in ${file.path}`;
+      default:
+        return `Discarding hunk in ${file.path}`;
+    }
+  }
+
+  async function applyLinePatch(action: PatchApplyAction, patch: string) {
+    if (!activeRepoPath || !linePatchFile || isBusy) return;
+    const file = linePatchFile;
+    operation = patchOperationLabel(action, file);
+    errorMessage = "";
+    linePatchError = "";
+
+    try {
+      applyStatus(await applyFilePatch(activeRepoPath, file.path, patch, action));
+      await refreshExplorerFiles(activeRepoPath);
+      await refreshFileHistory(activeRepoPath);
+
+      const updatedPatch = await getFilePatch(activeRepoPath, file.path, linePatchStaged);
+      if (updatedPatch.trim()) {
+        linePatchText = updatedPatch;
+      } else {
+        linePatchOpen = false;
+        linePatchFile = null;
+        linePatchText = "";
+      }
+    } catch (error) {
+      linePatchError = errorToMessage(error);
+      errorMessage = linePatchError;
+    } finally {
+      operation = "";
+    }
+  }
+
   async function stageAllFiles() {
     const paths = changedFiles.filter((f) => f.unstaged !== null).map((f) => f.path);
     if (paths.length === 0) return;
@@ -1129,6 +1210,7 @@
             onStage={stageFile}
             onUnstage={unstageFile}
             onDiscard={discardFile}
+            onPatch={openLinePatch}
             onStageAll={stageAllFiles}
             onUnstageAll={unstageAllFiles}
           />
@@ -1189,6 +1271,20 @@
   />
 {/if}
 
+{#if linePatchOpen && linePatchFile}
+  <LinePatchDialog
+    file={linePatchFile}
+    staged={linePatchStaged}
+    patch={linePatchText}
+    {isBusy}
+    isLoading={linePatchLoading}
+    error={linePatchError}
+    onClose={closeLinePatch}
+    onRefresh={refreshLinePatch}
+    onApply={applyLinePatch}
+  />
+{/if}
+
 {#if globalSearchOpen}
   <GlobalSearchDialog
     {hasRepository}