UI redesign: split-pane roster/editor/inspector, filters, tabs, and shortcuts

[martydill]

Motivation

• Restructure the editor UI into a multi-pane layout to keep the party roster visible while editing a single character.
• Surface derived stats and validation in a dedicated inspector to reduce modal popups and improve safety.
• Add quick filters, search, and keyboard shortcuts to speed common workflows.
• Provide a maintainable tabbed editor structure to make adding fields and tabs easier.

Description

• Reworked main window layout to nested SplitContainer panels with a left party roster, central tabbed editor (TabControl) and right inspector panel, and added a MenuStrip, ToolStrip, and StatusStrip.
• Added roster search and race/class filter UI backed by a new _allRecords list and ApplyRosterFilter() to populate the visible _records binding list.
• Introduced selection safety via ConfirmSelectionChange()/SelectSlot() and improved save/dirty UX with UpdateWindowTitle(), UpdateStatus(), MarkDirty(), and writing saves from _allRecords.
• Added inspector controls (_derivedStatsLabel, _validationList) and RefreshInspector() to compute/display derived values and validation issues, plus portrait preview improvements and typed _portraitFramesByFile.
• Implemented keyboard shortcuts in ProcessCmdKey for Ctrl+S, Ctrl+Shift+S, and Ctrl+F, modularized UI construction into Build* helpers, and added CHARACTER_ROSTER_EDITOR_UI_REDESIGN.md documentation describing the redesign.

Testing

• Built the solution with dotnet build and the build completed successfully.
• Ran dotnet test against the repository and no automated unit tests were found (no failures reported).

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Codex Task