Queue selection and split add-button improvements from build plate feedback

[jaak0b]

Fixes #15

Queue usability changes from the issue's feedback:

• The whole queue row now toggles selection; editing moved to an explicit pencil button, so a missed checkbox click no longer opens the editor and loses your place.
• Shift-click selects or deselects a contiguous range of rows; the clicked row's new state is applied to the whole range. Checkboxes participate too.
• Drawer group headers select or deselect all their plates on click; opening the drawer editor moved to a pencil button.
• The add buttons on every tab became a split button: the primary action queues as before, and a dropdown offers "Add to newest build plate" (creates one if none exists) plus direct STL/3MF downloads of the configured part without queueing.

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