Bump riparion-editor: ↓ moves into the autocomplete list

#39 in Riparion/riparion-cms — merged 2026-06-02

Pin bump to pick up Riparion/riparion-editor#5, which refines the [[/ autocomplete keyboard model:

• Popup opens with nothing highlighted; the first ↓ moves into the list on the first item (↑ on the last), then ↓/↑ navigate.
• Enter/Tab insert only once an item is selected; with nothing selected, Enter is a normal newline (you must ↓ first). After accept, arrow keys return to normal editing.
• Esc closes the menu with no insert.

No host code change — MarkdownPane forwards the now-Option<usize> selected() to CompletionPopup unchanged. Only Cargo.lock moves (d819a14 → d6372ed).

Verification

clippy -D warnings on server,sqlite and wasm/web — clean. (Behavior covered by the crate's tests + manual steps in TODO_VERIFICATION.md.)

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