Add multi-tab graph pages with per-tab nodes/connections, context menu, and inline rename

[Copilot]

Adds a tab bar below the ribbon so nodes can be organized across named pages. Each tab owns independent node and connection collections. Tabs support right-click context menu (new tab to the right, rename, delete), inline rename that auto-activates on tab creation (commits on Enter/Escape/focus-loss), and a "+" button that sits immediately to the right of the last tab.

Architecture

• TabViewModel — new VM holding Name, IsEditing, Nodes, Connections per tab
• MainWindowViewModel.Nodes / .Connections — now computed properties delegating to ActiveTab; OnActiveTabChanged clears selection and notifies bindings
• Tab commands — AddTabCommand, AddTabAfterCommand, DeleteTabCommand (last tab protected); NewGraph resets to a single "Main" tab
• Auto-rename — AddTab and AddTabAfter set IsEditing = true immediately; a FocusActiveTabRenameBox() helper defers focus and select-all to the rename TextBox after Avalonia renders the new item

Serialization

New multi-tab JSON format via TabData DTO (title, nodes, layout, connections) nested under a tabs array. Legacy single-tab files (no tabs key) deserialize into a single tab automatically.

{
  "tabs": [
    { "title": "Main", "nodes": [...], "layout": {...}, "connections": [...] },
    { "title": "Tab 1", "nodes": [...], "layout": {}, "connections": [] }
  ]
}

UI

Tab bar is a ScrollViewer wrapping a horizontal StackPanel containing the tab ListBox followed directly by the "+" button, so the button always appears immediately after the last tab and scrolls with them. Each tab item's DataTemplate contains a read-mode TextBlock and an edit-mode TextBox toggled by IsEditing. Context menu actions dispatch to MainWindowViewModel commands via code-behind handlers.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Tabs</issue_title>
<issue_description>I want the graph to have tabs, such that nodes can be places on pages that represent the contents of the tab.
I want a tab bar at the top, below the ribbon and above the graph, and a "+" button on the right side of the tabs area to create new tabs with.
You should be able to right click and get options to create a new tab (to the right of the selected tab), rename the selected tab, or delete the selected tab. Selected doesn't mean current, it means the one right-clicked.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Tabs #27

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[lunarcloud]

@copilot three notes:

• I want the new tab button to be just to the right of the last tab.
• when a new tab is created, automatically go into renaming mode
• the first tab should be named "Main" not "Tab 1"

[Copilot]

@copilot three notes:

• I want the new tab button to be just to the right of the last tab.
• when a new tab is created, automatically go into renaming mode
  ...

Done in commit 1a6af74:

• "+" button position — Restructured the tab bar to use a ScrollViewer + StackPanel so the button sits immediately after the last tab (and scrolls with them). Sam's NotoSymbolsFont styling is preserved.
• Auto-rename on new tab — AddTab and AddTabAfter now set IsEditing = true on the new tab and call a deferred FocusActiveTabRenameBox() helper that focuses and selects the rename TextBox once Avalonia has rendered it. The context-menu Rename action also auto-focuses.
• First tab named "Main" — Default tab in the constructor and NewGraph reset are now "Main" instead of "Tab 1".

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