feat: bend curves by dragging and move segments once selected

[kostyafarber]

Summary

• Dragging an unselected cubic segment now bends it without a modifier. Shift still snaps the handles to 45° steps and Alt keeps each handle's direction.
• A segment translates only once it is selected: click it, then drag. Straight lines have nothing to bend, so dragging them still translates.
• Cmd no longer affects bending. It keeps only the Cmd-hover upgrade preview and the Cmd-click upgrade on straight lines, so Cmd-hover over a cubic no longer shows the bend cursor.
• The bend cursor follows the bending state, so it shows for the whole drag.

Risks

• This narrows Align select and pen gestures with standard outline editing #465's bounding-box rule. An unselected cubic inside the selection's bounding box now bends instead of moving the selection. Unselected points, anchors, lines, and components inside the box still move the selection.
• Alt-drag on an unselected cubic now bends with each handle's direction kept. Before, it duplicated the current selection.
• A segment counts as selected only when the segment itself is selected. If both of its end points are selected but the segment is not, dragging it still bends it.

Issue

Closes #472

Testing

• pnpm check (typecheck, lint, 1189 vitest tests) passed
• Updated unit tests: plain-drag bend keeps the end points fixed, click-then-drag translates, bend lifecycle, Shift/Alt modifiers, undo/redo and cursor tests no longer hold Cmd
• Updated e2e/editor.spec.ts "keeps custom cursors on the canvas while hovering and bending across sidebars" for the plain-drag bend. Not run locally; left to PR CI
• Not yet verified by hand in the running app

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