Extended markdown, Mermaid diagrams, and Typefully integration

New

• Typefully integration — draft social posts in Cogito's editor, push to Typefully, pull edits back, and check draft status without leaving the app. Head to the app settings to get started.
• Callouts, footnotes, and definition lists — extended markdown syntax renders in preview, source highlighting, and export, with same-note navigation for heading anchors and footnote references.
• Mermaid diagrams — fenced mermaid blocks render as diagrams in preview and PDF export.
• Task progress in the sidebar — notes and favorites now show a progress ring reflecting task completion, based on the markdown tasks contained in the notes. Turn it off in Settings for a quieter sidebar.
• Quick Open button in the sidebar footer for fast keyboard-free access alongside ⌘ P.

Improved

• Preview rendering — refined code blocks, polished callout styling, and more consistent spacing between markdown blocks.
• Cursor behavior — the text caret stays inside the editor surface; links show a pointing hand in both source and preview.

Fixed

• PDF export no longer hangs (~19 seconds) on notes with code blocks.
• First keystrokes in new notes are no longer lost during the initial editor settle.
• Case-only renames (note.md → Note.md) work on default macOS filesystems.
• Untitled notes autosave without prematurely locking in half-typed filenames.
• Task list continuation stays indented after pressing Return twice inside a list item.
• Sidebar toggle clicks and back/forward history reset correctly on launch.
• Wrapped footnotes align properly and nested code blocks in lists render as separate blocks.