Explorer ergonomics: resizable column, collapsible sidebar, file preview pane

[dpetreleyai]

Problem

Follow-up to #49. Three ergonomic gaps once the explorer column is in daily use:

1. the column has a fixed 230px width — too narrow for deep trees, wasted space for shallow ones;
2. the project sidebar can't get out of the way — with the explorer open, two fixed columns eat 480px before the terminal starts;
3. clicking around the tree gives no way to see a file — you still open an editor to check what a script or CLAUDE.md contains.

Intended fix

• Resizable explorer: a drag handle on the column edge (min 140 / max 520px), width persisted in UI settings.
• Collapsible sidebar: a chevron collapses the project sidebar to a slim rail and back; state persisted. Everything stays one click away.
• Context preview pane: selecting a file in the explorer shows its content read-only in a pane above the terminal (existing read_file command; text files only, size-capped, non-text shows a hint). Header shows the file name plus buttons to insert the path into the terminal and to close. Single click = preview; double click = insert path (the old single-click action); right-click = open in OS, unchanged.

All persisted in UI settings, i18n EN/NL, no new write operations.

[astetic-dev]

Acknowledged - deliberately not part of 0.4.0. Still under consideration for a future release; keeping it open on the backlog.