Fix widget size (zoom) breaking grid snap and containment in the design editor

[Salvialf]

• _plan.css.zoom could be stored as an empty string when the "Taille du widget" field was saved while left blank. init(value, default) only treats undefined/null as unset, not '', so zoomScale ended up NaN for these widgets, breaking both grid snap and containment entirely. Now explicitly falls back to 1 when the parsed value is NaN.
• Reworked the zoom-compensated drag handling so grid snap keeps working correctly for widgets with an explicit size other than 1, and so dragging one now keeps it within the design bounds (previously fully disabled as soon as the size differed from 1).
• Added the same kind of correction to resizing: a zoomed widget resized past the design's edge is now brought back within bounds once the resize ends, instead of being left overflowing.
• Removed the now-outdated tooltip warning on "Taille du widget", since the placement/grid-snap issues it warned about are fixed.

Closes #3438