Desktop UI and service for split into dependent branches

- Added menu items and supporting logic for 'Split into dependent branch' in FileContextMenu.svelte.
- Added splitBranchIntoDependentBranch support to stackService (API, handler, mutation mapping).
This commit covers desktop app changes for the new feature, separate from backend changes.

Author: estib-vega

apps/desktop/src/components/v3/FileContextMenu.svelte

@@ -63,6 +63,7 @@
 	const [branchChanges, branchingChanges] = actionService.branchChanges;
 	const [absorbChanges, absorbingChanges] = actionService.absorb;
 	const [splitOffChanges] = stackService.splitBranch;
+	const [splitBranchIntoDependentBranch] = stackService.splitBrancIntoDependentBranch;
 
 	const userSettings = getContextStoreBySymbol<Settings, Writable<Settings>>(SETTINGS);
 	const isUncommitted = $derived(selectionId.type === 'worktree');
@@ -243,6 +244,36 @@
 			newBranchName: newBranchName.data
 		});
 	}
+
+	async function splitIntoDependentBranch(changes: TreeChange[]) {
+		if (!stackId) {
+			toasts.error('No stack selected to split off changes.');
+			return;
+		}
+
+		if (selectionId.type !== 'branch') {
+			toasts.error('Please select a branch to split off changes.');
+			return;
+		}
+
+		const branchName = selectionId.branchName;
+
+		const fileNames = changes.map((change) => change.path);
+		const newBranchName = await stackService.newBranchName(projectId);
+
+		if (!newBranchName.data) {
+			toasts.error('Failed to generate a new branch name.');
+			return;
+		}
+
+		await splitBranchIntoDependentBranch({
+			projectId,
+			sourceStackId: stackId,
+			sourceBranchName: branchName,
+			fileChangesToSplitOff: fileNames,
+			newBranchName: newBranchName.data
+		});
+	}
 </script>
 
 <ContextMenu bind:this={contextMenu} rightClickTrigger={trigger}>
@@ -332,6 +363,13 @@
 								contextMenu.close();
 							}}
 						/>
+						<ContextMenuItem
+							label="Split into dependent branch"
+							onclick={async () => {
+								await splitIntoDependentBranch(changes);
+								contextMenu.close();
+							}}
+						/>
 					{/if}
 				{/if}
 			</ContextMenuSection>

apps/desktop/src/lib/stacks/stackService.svelte.ts

@@ -808,6 +808,10 @@ export class StackService {
 		return this.api.endpoints.splitBranch.useMutation();
 	}
 
+	get splitBrancIntoDependentBranch() {
+		return this.api.endpoints.splitBranchIntoDependentBranch.useMutation();
+	}
+
 	stackDetailsUpdateListener(projectId: string) {
 		return this.api.endpoints.stackDetailsUpdate.useQuery({
 			projectId
@@ -1538,6 +1542,26 @@ function injectEndpoints(api: ClientState['backendApi']) {
 					invalidatesList(ReduxTag.Stacks)
 				]
 			}),
+			splitBranchIntoDependentBranch: build.mutation<
+				{ replacedCommits: [string, string][] },
+				{
+					projectId: string;
+					sourceStackId: string;
+					sourceBranchName: string;
+					newBranchName: string;
+					fileChangesToSplitOff: string[];
+				}
+			>({
+				extraOptions: {
+					command: 'split_branch_into_dependent_branch'
+				},
+				query: (args) => args,
+				invalidatesTags: (_result, _error, args) => [
+					invalidatesItem(ReduxTag.StackDetails, args.sourceStackId),
+					invalidatesItem(ReduxTag.BranchChanges, args.sourceStackId),
+					invalidatesList(ReduxTag.Stacks)
+				]
+			}),
 			stackDetailsUpdate: build.query<void, { projectId: string }>({
 				queryFn: () => ({ data: undefined }),
 				async onCacheEntryAdded(arg, lifecycleApi) {