Previously always silently wrote to Downloads with a fixed name — no
way to pick where it goes or rename it. Added
exportConversationWithSavePanel(format:defaultFilename:), which shows
a native NSSavePanel (defaulting to Downloads + the same filename as
before) before writing. Wired into all three File menu export buttons.
The /export slash command and the conversation list's per-row Export
submenu are unchanged (still write straight to Downloads) — the slash
command already takes an explicit filename argument inline, and the
row export is meant as a quick one-click action rather than a
save-as workflow.