Add auto_preview setting for Markdown, SVG, and CSV files

[anuragxxd]

Closes #10966

Summary

Adds an auto_preview setting under preview_tabs that automatically opens a preview when opening previewable files (Markdown, SVG, and CSV). The setting accepts three values:

• "off" (default) — no automatic preview
• "same_pane" — opens the preview in the same pane, replacing the editor tab
• "to_side" — opens the preview in an adjacent pane to the side

{
  "preview_tabs": {
    "auto_preview": "same_pane"
  }
}

Demo

zed-auto-preview-demo.mov

Approach

Each preview crate (markdown_preview, svg_preview, csv_preview) subscribes to workspace::Event::ItemAdded. When a new editor is added:

1. Check the auto_preview setting
2. Verify the file is previewable (by extension for Markdown/SVG/CSV)
3. Use window.defer to create the preview view after the editor item is fully mounted

Using window.defer and calling preview creation functions directly (bypassing the action system) avoids both the dispatch timing issue from #48733 and reentrancy panics from nested pane.add_item calls. For to_side mode, the code falls back to any existing non-active pane before attempting split_pane, since find_pane_in_direction depends on bounding boxes that may not exist yet in the deferred context.

Changes

• Setting: 3-variant AutoPreviewMode enum (Off, SamePane, ToSide) in PreviewTabsSettingsContent
• Markdown: auto-preview via file extension (.md, .markdown)
• SVG: auto-preview via file extension (.svg)
• CSV: auto-preview via file extension (.csv), gated behind TabularDataPreviewFeatureFlag
• Settings UI: dropdown in the Preview Tabs section (registered in SettingFieldRenderer)
• VSCode import: maps markdown.extension.preview.autoShowPreviewToSide → auto_preview: "to_side"
• Documentation: all-settings.md and default.json updated
• Tests: 3 integration tests (same_pane creates preview, off doesn't, non-markdown is ignored)

Context

This is a resubmission of #56950 with fixes for the settings UI renderer registration ("NO RENDERER" issue) and a to_side pane resolution bug. Built and tested end-to-end on macOS 15 with Xcode 26.

Prior art: #44346, #48733. Addresses all review feedback from @SomeoneToIgnore's #48733 review cycle.

Release Notes:

• Added auto_preview setting to automatically open a preview when opening Markdown, SVG, or CSV files

[anuragxxd]

@SomeoneToIgnore CI is green now. I also built this locally on macOS and tested both same_pane and to_side end to end. The demo in the PR description shows the fixed settings UI and preview behavior. Would appreciate another look when you have time.

[SomeoneToIgnore]

Thank you.
Now, we need to consider the usability: for that, you have to run the app and use it a bit with various cases.

Here's what I see as totally not acceptable:

slop.mov

Please, open the editor or IDE of your choice and compare with this behavior.
For example, VSCode:

has_some_thought_in_it.mov

I see at least the following UX discrepancies:

• Auto preview actually opens when I open another file
• Auto preview does not get the focus, it's the editor that keeps it
• Auto preview gets replaced instead of duplicating

Let's fix this and apply more rigorous manual testing first, then continue.