Enforce fixed size for Todo View dialog and add sizing tests

[multiplex55]

Motivation

• Ensure the "View Todos" dialog has a deterministic fixed size to prevent UI stretching and clipping when contents change.
• Make the list scroll area height deterministic so the dialog does not grow with content and the Close/filter rows remain visible.

Description

• Introduce sizing constants TODO_VIEW_SIZE and TODO_VIEW_LIST_HEIGHT and a small helper todo_view_layout_sizes() in src/gui/todo_view_dialog.rs to centralize layout values.
• Enforce a fixed window size by using .resizable(false) and setting .default_size, .min_size, and .max_size to the fixed TODO_VIEW_SIZE values in the egui::Window::new("View Todos") builder.
• Replace the ui.available_height() driven scroll max height with the deterministic TODO_VIEW_LIST_HEIGHT so the ScrollArea is bounded and the window remains fixed.
• Re-export the layout helper from src/gui/mod.rs and add tests: a unit test in the dialog module asserting the constants via todo_view_layout_sizes() and an integration-style GUI test in tests/todo_plugin.rs that opens the dialog and asserts the rendered window size matches the fixed size.

Testing

• Added unit test todo_view_layout_sizes_constants in src/gui/todo_view_dialog.rs which asserts TODO_VIEW_SIZE and TODO_VIEW_LIST_HEIGHT and their relationship; this test was added but not executed here.
• Added GUI integration test todo_view_dialog_has_fixed_size in tests/todo_plugin.rs that opens the dialog and asserts the window rect size equals the fixed layout; this test was added but not executed here.
• No automated test suite was run as part of this change (tests were added only).

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Codex Task