Description
Ask for the filename up front in a centered dialog whenever a new file is created, and show the filename in editor mode as an uneditable header bar styled like preview's title bar. Collapses save-as/new-file into one dialog component (new internal/dialog package) and reuses it for F2 rename. Design: docs/superpowers/specs/2026-07-30-new-file-dialog-design.md
Acceptance Criteria
- #1 Ctrl+N/Ctrl+B, picker Ctrl+N (prefilled with query), and 'glint -n' with no name all open a centered naming dialog
- #2 Enter creates the file immediately and opens it; existing name opens that file; slashes nest subdirs; empty name reports 'Type a name first'
- #3 Esc cancels back to the previous mode with nothing written
- #4 Editor mode shows the basename (minus .md) as a full-width header bar in the top pad, costing no text rows
- #5 Pathless buffer shows muted 'Untitled'; dirty buffer appends a bullet
- #6 F2 renames the current file on disk, rebinds the path, and reopens Harper under the new URI
- #7 ModeSaveAs/saveInput/saveBar removed; Ctrl+S on a pathless buffer opens the same dialog