revert: remove fork-only changes from release sync

Revert payment/wechat, sora/claude-max cleanup, fork-only migrations,
and cosmetic changes that were brought in by the release sync commit.
Keep only channel-monitor related improvements:
- PublicSettingsInjectionPayload named struct with drift test
- ChannelMonitorRunner graceful shutdown in wire
- image_output_price in SupportedModelChip
- Simplified buildSelfNavItems in AppSidebar
- Gateway WARN logs for 503 branches

backend/internal/handler/admin/account_handler_mixed_channel_test.go

@@ -111,7 +111,7 @@ func TestAccountHandlerCreateMixedChannelConflictSimplifiedResponse(t *testing.T
 	var resp map[string]any
 	require.NoError(t, json.Unmarshal(rec.Body.Bytes(), &resp))
 	require.Equal(t, "mixed_channel_warning", resp["error"])
-	require.Contains(t, resp["message"], "claude-max")
+	require.Contains(t, resp["message"], "mixed_channel_warning")
 	_, hasDetails := resp["details"]
 	_, hasRequireConfirmation := resp["require_confirmation"]
 	require.False(t, hasDetails)
@@ -140,7 +140,7 @@ func TestAccountHandlerUpdateMixedChannelConflictSimplifiedResponse(t *testing.T
 	var resp map[string]any
 	require.NoError(t, json.Unmarshal(rec.Body.Bytes(), &resp))
 	require.Equal(t, "mixed_channel_warning", resp["error"])
-	require.Contains(t, resp["message"], "claude-max")
+	require.Contains(t, resp["message"], "mixed_channel_warning")
 	_, hasDetails := resp["details"]
 	_, hasRequireConfirmation := resp["require_confirmation"]
 	require.False(t, hasDetails)