fix: unfocus UI inputs on viewport click, refuse duplicate id in ID input

[vincentfretin]

Two improvements to the panel/viewport interaction:

Unfocus UI inputs when clicking the viewport

raycaster.js called event.preventDefault() on every viewport mousedown, which cancels the browser's default focus transfer — so clicking the viewport never unfocused the currently focused panel input.

• Only preventDefault() non-left buttons now (still needed so middle-click zoom doesn't trigger the browser's autoscroll). Left-click performs the native focus transfer, so a focused input blurs — and commits its pending edit — before the selection changes.
• user-select: none added on the canvas, since preventDefault() was incidentally blocking drag-started text selection (drag-orbiting could otherwise select panel text).

Refuse duplicate ids in the ID input

Typing an id that already belongs to another element was silently committed, producing a duplicate id in the document, which breaks entity lookups done with getElementById/querySelector (selector-type component properties, links between entities, …).

changeId now refuses such a value: a message is shown below the input and the input is remounted so it displays the entity's current id again.

The id "aBox" is already used by another element, the previous id was restored.

Dead CSS cleanup

The .aframe-inspector-opened a-scene .a-canvas and .a-canvas.state-dragging rules in index.styl are nested under #aframeInspector, and the canvas is not a descendant of it, so they never matched (nothing in src adds the state-dragging class anyway). Removed; the new user-select rule lives under body.aframe-inspector-opened, which does match.

Tested

On the examples page with headless Chromium: viewport left-click blurs a focused id input; typing an existing id is refused with the message shown, and the message clears on entity switch; a valid rename still works and emits entityidchange; canvas user-select is none; middle/right-click still preventDefault.

Same change on the aframe-editor fork: c-frame#79

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