Refactor note graph dialog to use explicit canvas/details layout sizing

[multiplex55]

Motivation

• Make the graph canvas always act as the dynamic remainder area of the dialog and remove the implicit, nested available_width() sizing logic.
• Centralize panel sizing constraints so layout is deterministic and testable when the details panel is toggled or the window is resized.

Description

• Added centralized layout constants (MIN_CANVAS_WIDTH, DETAILS_WIDTH_RATIO, DETAILS_MIN, DETAILS_MAX, MIN_CANVAS_HEIGHT) and a pure helper fn compute_graph_layout(total_width: f32, details_visible: bool) -> (f32, Option<f32>) that returns (canvas_width, details_width) with clamping and minimum-enforcement logic.
• Replaced implicit horizontal sizing in NoteGraphDialog::ui with a single computation from ui.available_width() and ui.available_height(), then explicitly allocated the canvas and optional details panel with allocate_ui_with_layout using the computed sizes so the canvas is the remainder area.
• Updated main_canvas to accept a precomputed canvas_size: Vec2 and allocate exactly that size; adjusted right_panel to honor the exact allocated width.
• Preserved existing rendering and interaction logic (camera zoom/pan, node hit-testing, selection, actions) while making panel sizing deterministic.

Testing

• Ran cargo fmt -- src/gui/note_graph_dialog.rs which succeeded.
• Added unit tests covering compute_graph_layout(...) (table-driven cases) and a regression test details_visible_canvas_only_shrinks_by_computed_details_width to assert canvas shrinkage equals the computed details width; tests are in src/gui/note_graph_dialog.rs test module.
• Attempted to run cargo test compute_graph_layout, but test execution is blocked in this environment due to a Rust toolchain mismatch (rustc 1.87.0 vs a dependency requiring rustc 1.88.0).

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Codex Task