feat: subtle hover outline on the shape under the cursor (#261)

[vibhavkatre]

What

Every element now gives a hover affordance: in select mode, the shape under the cursor gets a subtle blue halo just outside its box, so hovering reads as interactive — mind-map nodes, flowchart nodes, block shapes, text.

How (low-risk, isolated)

A self-contained HoverOutline component that owns its own SVG pointer listener (mirrors MindmapHoverHandles) and renders a non-interactive halo — it never touches the core selection/pointer path. It:

• paints last in the viewport group (like the Mind-map node text editing: typed text is invisible until you click away (no live render) #258 editor fix) so an opaque shape painted by the whiteboard layer on the unified canvas can't occlude it, and draws the halo just outside the box for the same reason;
• is skipped for the selected shape (its selection outline shows instead) and under any non-select tool.

Scope

This is the hover-feedback half of #261. The add-node CTAs on hover and the refined selection border land with the handle rework (#265). #261 stays open.

Tests

• Guards: outline gated to unselected + select-tool; tracks topmost interactable shape; painted after the editor (occlusion-proof).
• Full suite green (723) + build clean.

Part of #261.

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