refactor(app): retire logo textures through the deferred-destroy queue

ensureLogoTexture() still freed the chat emoji, header logo and coin logo
textures it replaced at once, while every other theme-driven replacement
already goes through retireTexture() and is freed at the next preFrame().
It runs at the top of render(), so nothing referenced them yet; this is
hygiene, so that the same rule covers every replaced texture.

- ensureLogoTexture(): the three direct DestroyTexture() calls become
  retireTexture(). Cost: the old textures live one extra frame on a theme
  or accent change.
- setGradientTexture() (unused today) retires the texture it replaces
  instead of dropping it.
- No drain at shutdown: the process exits and the live logo and gradient
  textures are not freed at exit either.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app.h

@@ -690,8 +690,8 @@ public:
     // Diagnostic string describing daemon memory detection path (for debugging)
     const std::string& getDaemonMemDiag() const { return daemon_mem_diag_; }
 
-    // Background gradient overlay texture
-    void setGradientTexture(ImTextureID tex) { gradient_tex_ = tex; }
+    // Background gradient overlay texture (a replaced texture is retired, freed at the next preFrame())
+    void setGradientTexture(ImTextureID tex) { if (tex != gradient_tex_) retireTexture(gradient_tex_); gradient_tex_ = tex; }
     ImTextureID getGradientTexture() const { return gradient_tex_; }
 
     // Logo texture accessor (wallet branding icon)