feat(myqbz): My QBZ rename/change-icon in Settings>Appearance (DQ3)

Phase-2 Slice 12a. Per the user's decision (DQ3), the "My QBZ" rename/change-icon
lives in Settings > Appearance, not a sidebar context menu:

- src/myqbz_prefs.rs: per-user {label, icon_path} persisted to
  users/<id>/myqbz_branding.json (scoped like plex_settings/tray_settings);
  init_for_user/seed/set_label/pick_icon(rfd)/reset_icon. Blank label coerces to
  "My QBZ"; a missing/deleted custom icon falls back to the default my-qbz.svg
  without crashing (path preserved). 4 unit tests.
- ui/settings/AppearanceSettings.slint: a "My QBZ" SettingRow (icon preview +
  label LineEdit + Change icon... + Reset).
- ui/shell/Sidebar.slint: the My QBZ row binds label/icon to MyQbzBrandingState
  (raw-icon flag renders a custom user image untinted; default glyph stays tinted).
- Seeded on enter_shell.

Backend crates untouched. cargo check -p qbz-slint green.

cc

crates/qbz-slint/ui/settings/AppearanceSettings.slint

@@ -14,7 +14,7 @@ import { Theme } from "../foundation/semantic-colors.slint";
 import { ThemeState } from "../foundation/theme-state.slint";
 import { Typography } from "../foundation/typography.slint";
 import { Radius } from "../foundation/radius.slint";
-import { AppearanceState, SidebarState, ShellState, ToastState } from "../state.slint";
+import { AppearanceState, SidebarState, ShellState, ToastState, MyQbzBrandingState } from "../state.slint";
 import { QbzToggle } from "../primitives/QbzToggle.slint";
 import { QbzSelect } from "../primitives/QbzSelect.slint";
 import { QbzIcon } from "../primitives/QbzIcon.slint";
@@ -298,6 +298,64 @@ export component AppearanceSettings inherits VerticalLayout {
             }
         }
     }
+    // "My QBZ" nav branding — rename + change-icon, re-homed from the Tauri
+    // sidebar context-menu modal (DQ3). The label input commits on edit
+    // (blank coerces to "My QBZ" in Rust); the icon picker / reset persist a
+    // custom image path. The sidebar "My QBZ" row reflects both live.
+    SettingRow {
+        label: @tr("My QBZ");
+        description: @tr("Rename the My QBZ hub and pick a custom icon. Leave the name blank to restore the default.");
+        HorizontalLayout {
+            spacing: 8px;
+            // Icon preview — the user image as-is (custom) or the default
+            // branded glyph tinted to text colour (matches the picker preview
+            // being a raw image, not the sidebar's accent mask).
+            Rectangle {
+                width: 34px;
+                height: 34px;
+                border-radius: Radius.sm;
+                border-width: 1px;
+                border-color: Theme.border-subtle;
+                background: Theme.surface-elevated;
+                if MyQbzBrandingState.has-custom-icon: Image {
+                    source: MyQbzBrandingState.custom-icon;
+                    image-fit: contain;
+                    width: 22px;
+                    height: 22px;
+                }
+                if !MyQbzBrandingState.has-custom-icon: QbzIcon {
+                    source: @image-url("../assets/icons/my-qbz.svg");
+                    width: 18px;
+                    height: 18px;
+                    tint: Theme.text-secondary;
+                }
+            }
+            // Label input — commits on edit; the placeholder is the default.
+            VerticalLayout {
+                alignment: center;
+                width: 150px;
+                name-edit := LineEdit {
+                    text: MyQbzBrandingState.label;
+                    placeholder-text: @tr("My QBZ");
+                    edited(s) => {
+                        MyQbzBrandingState.set-label(s);
+                    }
+                }
+            }
+            SecondaryButton {
+                label: @tr("Change icon...");
+                clicked => {
+                    MyQbzBrandingState.pick-icon();
+                }
+            }
+            SecondaryButton {
+                label: @tr("Reset");
+                clicked => {
+                    MyQbzBrandingState.reset-icon();
+                }
+            }
+        }
+    }
     SettingRow {
         label: @tr("Show playlist cover collages in sidebar");
         description: @tr("Render a 2×2 thumbnail of track covers next to each playlist. Disable on low-end machines to skip the extra images.");