gui: ship the real icon set, wire toolbar/tray/window icon, rebase onto main

Adwaita-derived (GNOME Project, LGPL-3-or-CC-BY-SA), recoloured per light/dark
skin and laid out in IDM's toolbar positions per docs/03-gui-spec.md §1.
Licence and per-file provenance recorded in gui/resources/icons/LICENSE.

New IconTheme mirrors ThemeManager's colorSchemeChanged hook so a live
light/dark switch swaps every action's glyph, not just the QSS. Toolbar/menu
actions, the tray icon and the window icon all now use it instead of the
SP_ArrowDown placeholder. Install-path notes for PKG filed as R5 in
gui/docs/pkg-qa-requests-m1.md.

Also rebases lane/gui onto main (13 commits, no gui/ conflicts).

Verified: full build clean, all 13 gui ctest targets pass (including
gui_no_download_logic), unhappy-path DoD gate PASS against mockd
(--drop-connection).

Co-Authored-By: Claude Sonnet 5 <[email protected]>

gui/src/util/IconTheme.cpp

@@ -0,0 +1,27 @@
+#include "util/IconTheme.hpp"
+
+#include <QGuiApplication>
+#include <QStyleHints>
+
+namespace velox::gui {
+
+IconTheme::IconTheme(QObject *parent) : QObject(parent) {
+    connect(QGuiApplication::styleHints(), &QStyleHints::colorSchemeChanged, this,
+            &IconTheme::onColorSchemeChanged);
+}
+
+QIcon IconTheme::icon(const QString &name) {
+    const bool dark = QGuiApplication::styleHints()->colorScheme() == Qt::ColorScheme::Dark;
+    return QIcon(QStringLiteral(":/icons/%1/%2.svg")
+                     .arg(dark ? QStringLiteral("dark") : QStringLiteral("light"), name));
+}
+
+QIcon IconTheme::appIcon() {
+    return QIcon(QStringLiteral(":/icons/app/velox.svg"));
+}
+
+void IconTheme::onColorSchemeChanged() {
+    emit iconsChanged();
+}
+
+}  // namespace velox::gui

gui/src/util/IconTheme.hpp

@@ -0,0 +1,34 @@
+// Resolves action icons to the light/dark SVG variant matching the current
+// QStyleHints::colorScheme(), same trigger ThemeManager uses for the QSS skin. Lane GUI.
+// docs/03-gui-spec.md §1/§7, gui/resources/icons/LICENSE.
+
+#pragma once
+
+#include <QIcon>
+#include <QObject>
+#include <QString>
+
+namespace velox::gui {
+
+class IconTheme : public QObject {
+    Q_OBJECT
+
+  public:
+    explicit IconTheme(QObject *parent = nullptr);
+
+    /// `name` is a stem under gui/resources/icons/{light,dark}/, e.g. "add-url".
+    static QIcon icon(const QString &name);
+
+    /// The fixed full-colour glyph (window icon, tray icon) — same in both themes.
+    static QIcon appIcon();
+
+  signals:
+    /// Fired on a live light/dark switch; callers holding an icon(name) result should
+    /// re-fetch and re-apply it.
+    void iconsChanged();
+
+  private slots:
+    void onColorSchemeChanged();
+};
+
+}  // namespace velox::gui