Add popups: segment editor, copy menu, export, help and command line

- segment_editor: type start/end times; the exact moments they mean are
  shown live, and invalid edits keep the popup open.
- time_prompt: "I started / paused / ended the day at...".
- copy_menu: every copy preset with a preview of what will be pasted.
- export_dialog, still_working (for sessions left running very long),
  help (generated from the key tables) and the `:` command line.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/ui/mod.rs

@@ -13,6 +13,7 @@
 
 pub mod component;
 pub mod keymap;
+pub mod popups;
 pub mod terminal;
 pub mod theme;
 pub mod widgets;