Move Markdown demo into apps/ workspace crate; add Help menu + Document Properties dialog

#13 in Riparion/riparion-editor — merged 2026-06-08

Summary

• Promote the runnable demo from examples/markdown to its own riparion-mdedit crate under apps/markdown, wired into a new Cargo workspace. The app owns its renderer/widget deps (the library pins no markdown dialect) and only builds for web, so it's a separate crate rather than part of the root package's --all-targets set. CI clippies it against wasm; README and a new justfile document the layout.
• Rename the second menubar menu from About to Help (its item is now just "About").
• Move the YAML frontmatter Properties editor out of the inline document into a modal dialog (dx catalog dialog), opened from a new Edit ▸ Document Properties item; drop File ▸ Add properties. The leading frontmatter is hidden from the preview and edited in the dialog, with a create-frontmatter affordance when the doc has none.

Notes

• The Properties dialog opens two animation frames after selection. The menubar selects on pointerdown and the dialog primitive light-dismisses on the next outside pointerdown/focusin, so a synchronous open caught the tail of the same click and flashed shut. (AlertDialog, used by About, has no light-dismiss and was unaffected.)

Test plan

• just check passes locally (fmt, host clippy, tests, wasm clippy, mdedit wasm clippy).
• Manual: Edit ▸ Document Properties opens and stays open; inline edit / add / remove of scalar fields works; About dialog still works.

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