Canvas: Ctrl with another tool drags with the selection tool used last

[theironchef]

Fixes #270.

Problem: on a Cmd press with any tool outside the selection family, handle_input (crates/ui-egui/src/canvas.rs) switched to the Selection tool for the drag. In the reference app it's the selection tool used last. So with the Pen tool, after using Direct Selection, a Cmd-drag on an anchor moved the whole path instead of that anchor.

Change

• UiState gets last_selection_tool, default "selection". VectorcraftApp::select_tool sets it whenever Selection, Direct Selection or Group Selection becomes the tool. It's covered by the struct's #[serde(default)].
• The temporary switch uses it, falling back to Selection if the stored value isn't one of the three.
• The tool check is now canvas::is_selection_tool, which the canvas and select_tool share.

Tests

• New canvas::tests::cmd_with_another_tool_drags_with_the_selection_tool_used_last, a headless canvas test with Cmd held (via Event::ModifiersChanged) while the Pen tool is active:
  • with Direct Selection used last, the Cmd-drag is done by Direct Selection, moves only the rectangle's bottom-right anchor, and the Pen comes back;
  • with Group Selection or Selection used last, those are the tools used.
• With the old select_tool("selection"), the test fails (left: "selection" right: "directSelection").
• cargo test -p vectorcraft-ui-egui: 555 passed.
• cargo clippy -p vectorcraft-ui-egui --all-targets -- -D warnings (Rust 1.99): clean.
• cargo fmt --all -- --check: clean.
• Checked by hand in a release build on Windows with the real mouse and keyboard: I clicked a triangle with Direct Selection, pressed P, and Ctrl-dragged its anchor at (200, 120) to (240, 100). Direct Selection was active during the drag, only that anchor moved, and the Pen came back.

I didn't run the full cargo xtask ci (wasm step) here.

🤖 Generated with Claude Code

[bflatastic]

Thanks @theironchef, merged! I rebased it on main (it now sits with #279, whose canvas test landed right next to yours) and added a ROADMAP line ("Temporary tools"). I also checked it live in the app: with the Pen tool, a Cmd-drag on a corner moved just that anchor when Direct Selection was used last, moved the whole object when Selection was, and the Pen tool came back afterwards.