Adds a CodeMirror 6 spot editor to the right-rail file preview so users can
make quick edits in-app without leaving for an IDE. Entering edit mode is a
pure in-place swap of the read view — same fixed-height header, same gutter
geometry/typography (mirrors SourceView 1:1) so nothing shifts — toggled via
the Edit button, a bare `e` when the pane is hovered/focused, or the tab.
- Save path is transport-agnostic (writeDesktopFileText): local Electron IPC
or a new hardened POST /api/fs/write-text on the dashboard server (path
validation, parent-must-exist, regular-files-only, size cap, atomic
temp-file + os.replace), behind the existing auth middleware.
- Stale-on-disk guard re-reads before writing and offers overwrite vs
discard-and-reload instead of clobbering external/agent edits.
- VS Code-style modified dot on the tab; ⌘/Ctrl+S and ⌘/Ctrl+Enter save,
Esc cancels; GitHub highlight style matched to the read view's Shiki theme.
- Typing stays render-free (draft in a ref; dirty flips once at the boundary).
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