Add branch name validation for worktree creation (#17)

Prevents creation of worktrees with branch names that already exist.
Shows real-time validation with red error text when a duplicate branch
name is entered. Validates against both local and remote branches.

Co-authored-by: Claude <[email protected]>

Author: built-by-as

index.html

@@ -86,7 +86,8 @@ <h2 class="modal-title">New Session Configuration</h2>
         <div class="form-group" id="branch-name-group">
           <label class="form-label">Branch Name (optional)</label>
           <input type="text" id="branch-name" class="form-input" placeholder="e.g., feature/add-login" />
-          <span class="text-xs text-gray-400 mt-1 block">Custom branch name for worktree (will also be used as session name)</span>
+          <span id="branch-name-error" class="text-xs text-red-400 mt-1 hidden">Branch already exists</span>
+          <span id="branch-name-help" class="text-xs text-gray-400 mt-1 block">Custom branch name for worktree (will also be used as session name)</span>
         </div>
 
         <div class="form-group">

renderer.ts

@@ -221,6 +221,7 @@ async function loadAndPopulateBranches(
   selectedBranch?: string
 ): Promise<void> {
   const branches = await ipcRenderer.invoke("get-branches", directory);
+  existingBranches = branches;
   parentBranchSelect.innerHTML = "";
 
   if (branches.length === 0) {
@@ -758,6 +759,13 @@ ipcRenderer.on("session-created", (_event, sessionId: string, persistedSession:
   if (setupCommandsTextarea) {
     setupCommandsTextarea.value = "";
   }
+
+  // Reset validation state
+  const branchNameError = document.getElementById("branch-name-error");
+  const branchNameHelp = document.getElementById("branch-name-help");
+  branchNameError?.classList.add("hidden");
+  branchNameHelp?.classList.remove("hidden");
+  existingBranches = [];
 });
 
 // Handle session reopened
@@ -808,8 +816,44 @@ const localDescription = document.getElementById("local-description");
 const browseDirBtn = document.getElementById("browse-dir");
 const cancelBtn = document.getElementById("cancel-session");
 const createBtn = document.getElementById("create-session") as HTMLButtonElement;
+const branchNameInput = document.getElementById("branch-name") as HTMLInputElement;
+const branchNameError = document.getElementById("branch-name-error");
+const branchNameHelp = document.getElementById("branch-name-help");
 
 let selectedDirectory = "";
+let existingBranches: string[] = [];
+
+// Validate branch name
+function validateBranchName(): boolean {
+  const branchName = branchNameInput?.value.trim();
+
+  if (!branchName) {
+    // Empty branch name is allowed (it's optional)
+    branchNameError?.classList.add("hidden");
+    branchNameHelp?.classList.remove("hidden");
+    return true;
+  }
+
+  // Check if branch already exists
+  const branchExists = existingBranches.some(branch =>
+    branch === branchName || branch === `origin/${branchName}`
+  );
+
+  if (branchExists) {
+    branchNameError?.classList.remove("hidden");
+    branchNameHelp?.classList.add("hidden");
+    return false;
+  } else {
+    branchNameError?.classList.add("hidden");
+    branchNameHelp?.classList.remove("hidden");
+    return true;
+  }
+}
+
+// Add input event listener for branch name validation
+branchNameInput?.addEventListener("input", () => {
+  validateBranchName();
+});
 
 // Toggle skip permissions checkbox visibility based on coding agent
 codingAgentSelect?.addEventListener("change", () => {
@@ -914,6 +958,10 @@ cancelBtn?.addEventListener("click", () => {
   projectDirInput.value = "";
   selectedDirectory = "";
   parentBranchSelect.innerHTML = '<option value="">Loading branches...</option>';
+  branchNameInput.value = "";
+  branchNameError?.classList.add("hidden");
+  branchNameHelp?.classList.remove("hidden");
+  existingBranches = [];
 });
 
 // Create session button
@@ -931,6 +979,12 @@ createBtn?.addEventListener("click", () => {
     return;
   }
 
+  // Validate branch name doesn't already exist for worktree sessions
+  if (sessionType === SessionType.WORKTREE && !validateBranchName()) {
+    alert("Cannot create worktree: branch already exists");
+    return;
+  }
+
   const setupCommandsTextarea = document.getElementById("setup-commands") as HTMLTextAreaElement;
   const setupCommandsText = setupCommandsTextarea?.value.trim();
   const setupCommands = setupCommandsText