Location customization, inline renaming, and editor polish

Locations can now be renamed without changing the actual folder, notes can be renamed inline, and the editor gains highlight syntax and heading shortcuts.

New

Customize locations — rename how a location appears in Cogito without changing the folder on disk, via a dedicated Customize Location sheet.
Inline note renaming — rename notes directly in the sidebar without opening a dialog.
Highlight syntax — ==highlighted text== is now recognized and rendered in both source and preview.
Heading shortcuts — quickly set heading levels from the keyboard while editing.

Improved

Smoother window lifecycle — closing and reopening windows preserves your tab stack and restores the right content, and the app no longer flashes an empty window on launch.
Frontmatter layout — preview columns size more consistently around frontmatter blocks.

Fixed

Switching between notes in preview mode no longer causes flicker or stale content.
Note restoration on relaunch is more reliable across sessions.