fix(gui): Gen+Send and MD Only with empty context no-op silently

ROOT CAUSE: 3 mismatched names in the empty-context warning path:

1. _handle_generate_send set self.show_empty_context_warning_modal = True
   but render_empty_context_modal checks self.show_empty_context_modal.
   The modal never opened.

2. _handle_generate_send / _handle_md_only never set
   self._pending_generation_action, so the modal's 'Proceed Anyway'
   button always saw None and dispatched nothing.

3. After Proceed Anyway, _pending_generation_action was never reset,
   so subsequent empty-context calls would dispatch the wrong action.

FIX:
- gui_2.py:494,501: show_empty_context_warning_modal -> show_empty_context_modal
- gui_2.py:494,501: set _pending_generation_action before showing modal
- gui_2.py:5385: reset _pending_generation_action = None after dispatch

Tests: tests/test_gen_send_empty_context.py (5 cases) covers all 4 dispatch
paths (generate/md_only x proceed/skip) plus the happy path with context.

37/37 regression pass. No new ImGui scope errors (2 pre-existing unrelated).

src/gui_2.py

@@ -491,14 +491,16 @@ class App:
 
  def _handle_generate_send(self) -> None:
   if not self.ui_selected_context_files and not getattr(self, "_pending_proceed_generate", False):
-   self.show_empty_context_warning_modal = True
+   self._pending_generation_action = 'generate'
+   self.show_empty_context_modal = True
   else:
    self._pending_proceed_generate = False
    self.controller._handle_generate_send()
 
  def _handle_md_only(self) -> None:
   if not self.ui_selected_context_files and not getattr(self, "_pending_proceed_md_only", False):
-   self.show_empty_context_warning_modal = True
+   self._pending_generation_action = 'md_only'
+   self.show_empty_context_modal = True
   else:
    self._pending_proceed_md_only = False
    self.controller._handle_md_only()
@@ -5384,6 +5386,7 @@ def render_empty_context_modal(app: App) -> None:
   if imgui.button("Proceed Anyway", imgui.ImVec2(150, 0)):
    if app._pending_generation_action == 'generate': app.controller._handle_generate_send()
    elif app._pending_generation_action == 'md_only': app.controller._handle_md_only()
+   app._pending_generation_action = None
    imgui.close_current_popup()
   imgui.same_line()
   if imgui.button("Cancel", imgui.ImVec2(120, 0)):