feat: add selective line restoration from historical commits

Add a new Tauri command to produce a diff between the working file and a historical commit (get_file_restore_patch) and support a new apply action ("restore-lines") that validates and applies only text-line changes for a single regular file.

Behavior changes and constraints:
- Fetch a filtered reverse diff for a file in a commit so UI can display selectable lines from an older revision.
- Applying "restore-lines" verifies the target is a regular file, rejects binary/metadata patches, and ensures the patch only modifies the selected file.
- Restored lines are applied to the working tree without staging other changes; the index is preserved.
- The operation rejects stale patches or patches targeting the wrong file.

UI wiring:
- Compare dialog gets a "Restore lines…" action for applicable modified files and opens the line-patch dialog in restore mode.
- Line-patch dialog gains a restore mode (restoreCommit) with adjusted UI/rendering to pair removed/added lines, helper text, and dedicated "Restore selected" / "Restore hunk" actions.
- App integration handles fetching the restore patch, applying selected lines, and refreshing views.

Tests:
- Add tests covering correct behavior (preserve unstaged/staged changes and index) and guard cases (stale/wrong-file patches).

src/App.svelte

@@ -71,6 +71,7 @@
     cancelCodeSearch,
     cancelFileHistory,
     applyFilePatch,
+    getFileRestorePatch,
     createBranch,
     createTag,
     deleteBranch,
@@ -495,6 +496,8 @@
   let selectedDiffPath = "";
   let diffHighlightQuery = "";
   let pendingRestoreFile: { commit: GitCommit; file: GitCommitFile } | null = null;
+  let linePatchRestoreCommit = "";
+  let linePatchRestoreRepo = "";
   let linePatchOpen = false;
   let linePatchFile: GitFileStatus | null = null;
   let linePatchStaged = false;
@@ -5019,6 +5022,8 @@
 
   async function openLinePatch(file: GitFileStatus, staged: boolean) {
     if (!activeRepoPath) return;
+    linePatchRestoreCommit = "";
+    linePatchRestoreRepo = "";
     linePatchOpen = true;
     linePatchFile = file;
     linePatchStaged = staged;
@@ -5039,13 +5044,59 @@
     }
   }
 
+  async function openHistoricalLineRestore() {
+    if (!activeRepoPath || !comparison || comparison.to_hash || isBusy) return;
+    const file = comparison.files.find(item => item.path === selectedDiffPath);
+    if (!file || file.status !== "modified" || file.old_path) return;
+    linePatchRestoreCommit = comparison.from_hash;
+    linePatchRestoreRepo = activeRepoPath;
+    linePatchFile = { path: file.path, old_path: null, staged: null, unstaged: "modified" };
+    linePatchStaged = false;
+    linePatchText = "";
+    linePatchError = "";
+    compareDialogOpen = false;
+    fileHistoryDialogOpen = false;
+    globalSearchOpen = false;
+    linePatchOpen = true;
+    await refreshLinePatch();
+  }
+
+  async function restoreSelectedLines(patch: string) {
+    if (!linePatchFile || !linePatchRestoreCommit || isBusy) return;
+    const file = linePatchFile.path;
+    const repo = linePatchRestoreRepo;
+    const commit = linePatchRestoreCommit;
+    if (repo !== activeRepoPath) { linePatchError = "The active repository changed. Reopen the comparison."; return; }
+    operation = appLanguage === "de" ? "Ausgewählte Zeilen wiederherstellen" : "Restoring selected lines";
+    linePatchError = "";
+    try {
+      applyStatus(await applyFilePatch(repo, file, patch, "restore-lines"));
+      linePatchText = await getFileRestorePatch(repo, commit, file);
+      comparison = await diffFileAgainstWorkingTree(repo, commit, file);
+      await refreshRepositoryViews(repo, { branches: false, commits: false });
+      await refreshFileHistory(repo, file, true);
+    } catch (error) { linePatchError = errorToMessage(error); }
+    finally { operation = ""; }
+  }
+
   async function refreshLinePatch() {
     if (!activeRepoPath || !linePatchFile) return;
+    if (linePatchRestoreCommit) {
+      linePatchLoading = true;
+      linePatchError = "";
+      try { linePatchText = await getFileRestorePatch(linePatchRestoreRepo, linePatchRestoreCommit, linePatchFile.path); }
+      catch (error) { linePatchError = errorToMessage(error); }
+      finally { linePatchLoading = false; }
+      return;
+    }
     await openLinePatch(linePatchFile, linePatchStaged);
   }
 
   function closeLinePatch() {
     if (isBusy) return;
+    if (linePatchRestoreCommit) compareDialogOpen = !!comparison;
+    linePatchRestoreCommit = "";
+    linePatchRestoreRepo = "";
     linePatchOpen = false;
     linePatchFile = null;
     linePatchText = "";
@@ -5132,6 +5183,7 @@
   }
 
   async function applyLinePatch(action: PatchApplyAction, patch: string, scope: "hunk" | "lines") {
+    if (action === "restore-lines") { await restoreSelectedLines(patch); return; }
     if (!activeRepoPath || !linePatchFile || isBusy) return;
     const file = linePatchFile;
     const staged = linePatchStaged;
@@ -6681,6 +6733,7 @@
     <module.default
       file={linePatchFile}
       staged={linePatchStaged}
+      restoreCommit={linePatchRestoreCommit}
       patch={linePatchText}
       {isBusy}
       isLoading={linePatchLoading}
@@ -6947,6 +7000,7 @@
       restoreLabel={pendingRestoreFile ? (appLanguage === "de" ? "Datei wiederherstellen" : "Restore file") : ""}
       onClose={closeCompareDialog}
       onRestore={restorePreviewedCommitFile}
+      onRestoreLines={openHistoricalLineRestore}
       onSelectFile={selectDiffFile}
     />
   {/await}