Add quick capture and an Inbox

## Goal

Creating something in Cortext currently starts with a filing decision. That gets in the way when you just need to write something down.

Add quick capture backed by a normal Cortext document. It lands in an Inbox and can be organized later, without choosing a collection or a place in the page tree up front.

## Tasks

- [ ] Put Quick capture within reach from anywhere in the workspace, including a keyboard entry point.
- [ ] Add an Inbox for unorganized captures. Opening one should use the regular document editor.
- [ ] Let a capture be titled, moved into the page tree, or added to one or more collections without creating a second document.
- [ ] Make the capture flow usable in Cortext Desktop and at the mobile widths supported by the workspace.
- [ ] Once a capture has a place in the tree or a collection, remove it from Inbox but keep its content, history, links, and identity.

## Acceptance criteria

- A user can capture a note without choosing a parent page or collection.
- The new note appears in Inbox immediately and can be edited like any other Cortext document.
- Moving it or applying a collection trait does not duplicate the document.
- An organized capture no longer appears in Inbox.
- The flow works from the workspace on desktop and mobile layouts.

## Out of scope

- A browser extension or operating-system share target. Inbox provides the destination those entry points can use later.

Start from the workspace entry point and the regular document editor, then trace how documents are created, placed in the page tree, and assigned collections. Check the desktop workspace and supported mobile layouts before implementing the flow. Done means captures enter Inbox without a parent, remain the same document when organized, and disappear from Inbox after gaining a tree place or collection.

Written by the indexing model from the issue text.