Project navigation: search by filename and by content

[syncended]

Add search/navigation across the project so users can quickly jump to a saved request without browsing the sidebar tree manually.

Scope

Two search modes:

• By filename — fuzzy match against .http file names (and group/directory names) in the project tree.
• By content — substring/regex match against the body of .http files (URL, method, headers, body).

UX notes

• Triggered from the main TUI workspace; opens an overlay/palette over the Editor pane.
• Results list is keyboard-navigable; Enter opens the selected request in the Editor.
• Esc dismisses the overlay without clearing the user's query (consistent with the Esc-preserves-input rule used elsewhere in the TUI).

Out of scope

• Search across history (tracked separately in History of executed requests #12).
• Saving/recalling search queries.