added loading screen

src/ui/loading.ui

@@ -0,0 +1,69 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- SPDX-License-Identifier: MIT -->
+<!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
+<!--
+  What is on screen between the chooser closing and the main window appearing: the only window that
+  exists while the buffer is planned or a recording is read. Both of those run with no main loop, so
+  this window is painted once and then stays still — which is why it says what is happening in words
+  rather than only spinning, and why there is no progress bar: nothing can move it.
+
+  Deliberately undecorated (`deletable=0`, no title buttons): a close button that cannot be serviced
+  until the work finishes is a lie, and the work is not cancellable once the buffer is being planned.
+-->
+<interface>
+  <object class="AdwWindow" id="loading_window">
+    <property name="title" translatable="yes">udpscope</property>
+    <property name="default-width">420</property>
+    <property name="default-height">220</property>
+    <property name="resizable">0</property>
+    <property name="deletable">0</property>
+    <property name="content">
+      <object class="AdwToolbarView">
+        <child type="top">
+          <object class="AdwHeaderBar">
+            <property name="show-start-title-buttons">0</property>
+            <property name="show-end-title-buttons">0</property>
+            <property name="title-widget">
+              <object class="AdwWindowTitle">
+                <property name="title" translatable="yes">udpscope</property>
+              </object>
+            </property>
+          </object>
+        </child>
+        <property name="content">
+          <object class="GtkBox">
+            <property name="orientation">vertical</property>
+            <property name="spacing">14</property>
+            <property name="halign">center</property>
+            <property name="valign">center</property>
+            <property name="margin-start">24</property>
+            <property name="margin-end">24</property>
+            <property name="margin-top">12</property>
+            <property name="margin-bottom">24</property>
+            <child>
+              <object class="GtkSpinner">
+                <property name="spinning">1</property>
+                <property name="width-request">36</property>
+                <property name="height-request">36</property>
+              </object>
+            </child>
+            <child>
+              <object class="GtkLabel" id="loading_title">
+                <property name="wrap">1</property>
+                <property name="justify">center</property>
+                <property name="css-classes">title-4</property>
+              </object>
+            </child>
+            <child>
+              <object class="GtkLabel" id="loading_detail">
+                <property name="wrap">1</property>
+                <property name="justify">center</property>
+                <property name="css-classes">dim-label</property>
+              </object>
+            </child>
+          </object>
+        </property>
+      </object>
+    </property>
+  </object>
+</interface>

src/ui/udpscope-ui.gresource.xml

@@ -14,6 +14,7 @@
     <file preprocess="xml-stripblanks">help.ui</file>
     <file preprocess="xml-stripblanks">source-form.ui</file>
     <file preprocess="xml-stripblanks">source-chooser.ui</file>
+    <file preprocess="xml-stripblanks">loading.ui</file>
     <file preprocess="xml-stripblanks">main-window.ui</file>
     <file preprocess="xml-stripblanks">cursors-panel.ui</file>
     <file preprocess="xml-stripblanks">measures-panel.ui</file>