[EndlessCCL]

What this PR does

Replace md-editor-v3 with a Tiptap visual editor and CodeMirror Markdown source editor while preserving Markdown storage and existing editor bindings.

• Share toolbar controls across modes and synchronize caret, selection and formatting state.
• Support contextual link actions, tables, image cropping and code language selection.
• Unify previews and public rendering, including code copying and existing attachments.
• Normalize pasted spacing and emoji, handle guest image uploads without losing drafts, and report upload failures safely.
• Remove the legacy editor dependency and configuration.

Why

Improve visual Markdown editing while keeping source editing available. Resolve inconsistent mode switching, pasted content, image uploads and rendering across editor surfaces.

How to review

1. Type or paste formatted content, switch modes and check caret position, formatting and Markdown preservation.
2. Exercise links, tables, code blocks and image upload, paste, drop and crop.
3. Check light/dark themes, English/Chinese labels and narrow toolbars.
4. Test anonymous uploads with guest permissions enabled and disabled. Failed uploads should preserve the draft without Tiptap warnings.

Validation: 49 unit tests, 62 browser scenarios and the production build passed. Upload failure checks mock storage responses; guest authentication uses the real endpoint. External storage was not exercised.

Checklist

• Commits are signed off (git commit -s)
• Tests pass locally (pnpm build, pnpm editor:test, pnpm editor:test:browser)
• No database schema changes; no migration required
• No public API or environment variable changes; editor documentation updated
• No secrets, internal URLs, or team member names in the diff