Markdown editor for item notes

[brianorwhatever]

Summary

• New full-page markdown editor at /note/:itemId — one big textarea with an Edit/Preview toggle (react-markdown + remark-gfm), debounced autosave, and a save-status indicator.
• Reuses the existing items.description field (no schema change). New Convex query getItemForEditor loads the item with permission gating; updateItem now caps descriptions at 50k.
• The item-details modal's inline notes textarea is replaced by a tap-to-open rendered-markdown preview, so the dedicated page is the single editing surface.
• Pure helpers (MAX_NOTE_LENGTH, clampNote, shouldPersist) in src/lib/noteEditor.ts with unit tests.

Design notes

• Editing state uses the Convex query as the source of truth with a local override only while typing (draft: string | null), so a saved note resumes tracking the server value rather than holding stale text.
• Markdown preview renders without rehype-raw/dangerouslySetInnerHTML — pasted HTML/<script> is inert.
• In the current permission model there is no viewer-only role, so a non-editor on /note/:itemId gets "note not available" rather than an unreachable read-only branch.

Test Plan

• bun test scripts/ — 68 pass (incl. new note-editor tests)
• npx tsc -b clean; npx eslint clean on new files; vite build succeeds
• Manual: create item → open notes → type markdown → Saving→Saved → toggle Preview → Back → reopen modal shows rendered preview; reload persists
• Manual: paste <script>alert(1)</script>, Preview renders it inert
• Manual: visit /note/<bad-id> → "This note isn't available."

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