fix: replace unsupported prompt() with inline folder-create input

[Jehu]

Problem

In the Agent Zero WebUI, window.prompt() is blocked. Clicking New Folder (or New Subfolder in the context menu) throws:

Alpine Expression Error: prompt() is not supported.
Expression: "$store.chatOrganizer.showCreateFolder(null)"

so folders cannot be created at all.

Fix

Folder creation now uses an inline input row that mirrors the existing inline rename flow (editingId/editValue):

• webui/chat_organizer_store.js: new creating/createValue state; showCreateFolder(parentId) opens the inline input (focuses it, expands the parent folder); new finishCreate() (guarded against Enter+blur double-fire), cancelCreate(), isCreating().
• extensions/webui/sidebar-chats-list-start/chat_organizer.html: inline create row at the top of .co-tree-scroll reusing the co-rename-input styling. Enter = create, Escape = cancel, blur = create.

Both entry points work: the header New Folder button and the New Subfolder context-menu item.

Verification

• node --check passes on the store
• no remaining prompt() usage
• manually tested in the Agent Zero WebUI sidebar