fix(gui): replace self with app in render_theme_panel

render_theme_panel is a module-level function that takes app as its
parameter, but two lines still referenced 'self' (line 6373 and 6376).
The function was converted from a method (_render_theme_panel) to a
module-level function in the module_taxonomy_refactor_20260627 Phase 1.3
(commit 3dd153f7), but the self -> app substitution was missed.

Symptom: on every frame, render_theme_panel called imgui.begin('Theme', ...)
which pushed the Theme window onto the imgui stack. Then the
'getattr(self, ...)' raised NameError. The exception was swallowed by
_render_main_interface_result's try/except, but the imgui.end() call
at the end of the function was never reached. The Theme window stayed
pushed on the stack, and HelloImGui's auto-managed MainDockSpace asserted
'Missing End()' on every frame.

The bug was masked earlier by commit 71028dad, which fixed a stale
'from src.command_palette import' in render_main_interface. Before that
fix, render_main_interface aborted entirely every frame, so the Theme
window's never-reached end() was hidden behind a different error.

Bisect confirmed: disabling any other default-visible window left the
error; only disabling Theme made /api/gui_health report healthy=True.

Verification:
- tests/test_default_layout_install.py: 3/3 PASS (install behavior unchanged)
- tests/test_api_hooks_gui_health_live.py: 1/1 PASS (was failing)
- tests/test_command_palette_sim.py: 7/7 PASS
- tests/test_saved_presets_sim.py: 2/2 PASS

src/gui_2.py

@@ -6370,10 +6370,10 @@ def render_theme_panel(app: App) -> None:
   ch_ct, ctrans = imgui.slider_float("##ctrans", theme.get_child_transparency(), 0.1, 1.0, "%.2f")
   if ch_ct:
    theme.set_child_transparency(ctrans)
-  bg_enabled       = getattr(self, 'bg_shader_enabled', False)
+  bg_enabled       = getattr(app, 'bg_shader_enabled', False)
   ch_bg, new_bg    = imgui.checkbox("Animated Background Shader", bg_enabled)
   if ch_bg and new_bg != bg_enabled:
-   self.bg_shader_enabled = new_bg
+   app.bg_shader_enabled = new_bg
    gui_cfg = app.config.setdefault("gui", {})
    gui_cfg["bg_shader_enabled"] = new_bg
    if hasattr(app, "_flush_to_config"): app._flush_to_config()