LPM-138 Refactor Tui with composable modals and use canvas
Refactors the TUI (dashboard + kanban) to use a shared, composable modal system with canvas-based overlay rendering, while consolidating styling into internal/style and expanding issue interactions (e.g., comments).
Changes:
Introduces a new pkg/tui/modal system (manager/stack + multiple modal types) and overlays modals using Lipgloss compositor layers.
Refactors dashboard/kanban views and input handling to use the modal manager + focus manager instead of per-modal boolean state.
Consolidates TUI styling by moving from pkg/tui/styles to internal/style and updates components to use the new style package; adds a shared footer renderer.
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