[Feature]: Markdown workspace mode with split editor, live preview, outline, and wikilinks

[DenizAltunkapan]

Problem

Cloud Page will soon be able to preview Markdown files (#262), but previewing is read-only. There is no way to actually write or maintain notes and documents inside Vault Web. For a private workspace this is a gap: users have to edit Markdown somewhere else and re-upload, and there is no notebook or wiki experience over the files they already store.

Example: a user keeps project notes as Markdown in their cloud. They want to open a note, edit it with live preview, jump between headings, and link to a related note, all without leaving Vault Web.

Proposed solution

Add a Markdown workspace mode that turns Markdown files in the cloud into an editing and note-taking experience, not just a preview.

Scope:

• a split view with the editor on one side and live preview on the other
• save edits back to the cloud file, with autosave and a clear saved/unsaved indicator (reuse the unsaved-changes guard from [Feature]: Warn before leaving unsaved Cloud file edits #261)
• an outline panel generated from the document headings for quick navigation
• wikilink support (for example [[other-note]]) that resolves to other Markdown files in the workspace, so notes can link to each other
• open through the shared file viewer shell ([Feature]: Workspace file viewer for images, video, audio, and PDF with sidebar actions cloud-page#98) rather than a separate one-off view

Notes:

• Everything stays as plain Markdown files in the existing cloud storage. No new document store, no database format, so files remain portable.
• Real-time multi-user co-editing is explicitly out of scope for this issue; this is single-user editing of shared, portable files.

Acceptance criteria

• A Markdown file can be edited in the browser with a live preview of the rendered output.
• Edits are saved back to the same cloud file, with autosave and a visible saved/unsaved state.
• An outline of the document headings is shown and can be used to navigate.
• Wikilinks between Markdown files resolve and are navigable.
• The editor opens through the shared file viewer shell rather than a separate view.
• Leaving with unsaved changes warns the user.
• Files stay as plain Markdown in the existing storage, with no proprietary format.

Related: extends the Markdown preview (#262) and the unsaved-changes warning (#261); hosted by the workspace file viewer shell (Vault-Web/cloud-page#98).