Refactor: extract export commands; co-locate sigtype popup logic.

Two focused, behaviour-preserving moves:

1. `OpenSignalTypeDialog` + `ApplySignalTypeChoice` moved from
   `shell.cpp` to `screen_sigtype_modal.cpp` so the popup owns all of
   its logic instead of having its open/apply functions live in the
   shell file.

2. The `export` command extracted from `commands.cpp` to a new
   `commands_export.cpp` under a `Tui::RegisterExportCommands()`
   member. `RegisterCommands()` calls it at the end. File-local
   helpers (`csv_quote`, `pin_side`) move alongside in an anonymous
   namespace.

Establishes the pattern for future per-group splits: declare a
`Register<X>Commands()` member, define it in its own file, call it
from the orchestrator. Other groups stay in `commands.cpp` for now —
nothing else has grown large enough to warrant the split.

Sizes: shell.cpp 497 → 448, commands.cpp 846 → 675 (+ 191 for the
new commands_export.cpp). DESIGN.md updated.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/tui/tui.hpp

@@ -201,6 +201,9 @@ public:
 private:
     // Lifecycle (commands.cpp)
     void RegisterCommands();
+    // Per-file command-group registrators. Each adds entries to the
+    // `commands` map. Called from RegisterCommands().
+    void RegisterExportCommands();   // commands_export.cpp
 
     // Shell (shell.cpp)
     void Print(const std::string &line);