qml: Introduce adaptive sidebar/tab-bar shell driven by size class

[pseudoramdom]

Summary

Introduces an adaptive navigation shell - a single declarative tree of tabs that renders as a persistent sidebar at regular widths and a bottom tab bar at compact widths, swapping automatically at runtime based on the window size.

Gated behind the -DENABLE_TABVIEW_SHELL=ON CMake flag

Related to #689

Details

• SizeClass - a small singleton that turns the live window dimensions into one of two coarse buckets: compact (narrow) and regular (wide).
• Tab - represents a single navigation destination. Each tab carries a title, an icon
• TabSection - groups related tabs together. In the sidebar, sections render with a header above their tabs to provide visual grouping
• TabView - the adaptive shell. It accepts a tree of tabs (and sections of tabs) and decides at runtime whether to render them as a left sidebar or a bottom tab bar based on the window's size class

TabView API at a glance

TabView {
    TabSection {
        title: qsTr("Node")
        Tab { title: "Node";    iconSource: "..."; contentComponent: Component { NodeRunner {} } }
        Tab { title: "Peers";   displayModes: TabView.Sidebar; ... }
        Tab { title: "Network"; displayModes: TabView.Sidebar; ... }
    }
    TabSection {
        title: qsTr("Wallet")
        displayModes: TabView.Sidebar
        model: walletListModel
        delegate: Tab {
            required property string name
            title: name
            onSelected: walletController.setSelectedWallet(name)
            contentComponent: Component { WalletContainer {} }
        }
        Tab { title: "Add Wallet"; iconSource: "..."; actionOnly: true; onTriggered: addWalletPopup.open() }
    }
    Tab { title: qsTr("Settings"); pinToBottom: true; contentComponent: Component { NodeSettings {} } }
}

Screenshots

Sidebar

Tabbar

[pseudoramdom]

Work towards #689