Ideogram4 Prompt Builder: make bbox editor usable on mobile. Fixes issue #660

[RoyHammerlin]

What
Makes the bounding-box editor in the Ideogram 4 Prompt Builder fully usable on a mobile/touch browser, and gives newly created regions a sensible minimum size.

Why
The canvas interaction (draw / move / resize), the layers-menu row reorder, and the colour-swatch reorder were all wired to mouse events (mousedown → document mousemove/mouseup). Touch browsers synthesise mousedown/mouseup/ dblclick from a tap, so selecting a region and double-tapping to edit worked, but they do not emit a continuous mousemove stream during a finger drag — so dragging, resizing, and drag-to-draw silently did nothing on mobile. There was also no touch-action rule, so touches were consumed as page scroll / pinch-zoom.

Changes (all in web/js/ideogram4_prompt_builder.js)

• Canvas drag, layer-row reorder, and swatch reorder now use Pointer Events (pointerdown/pointermove/pointerup) with setPointerCapture, covering mouse, touch and pen through one path.
• touch-action: none on the canvas and on the draggable rows/swatches.
• Touch/pen double-tap on a region opens the inline editor; double-tap on empty space creates a new default-size region. Mouse keeps using the native dblclick handler.
• Long-press on the canvas opens the layers menu (the touch equivalent of right-click).
• New "+ Box" toolbar button — adds a region in the centre to drag/resize. Also covers the case where a full-canvas background region leaves no empty space to draw on.
• Newly drawn regions are grown to a minimum grabbable size; tap-sized "draws" are discarded rather than left as pixel-thin slivers. Sizes are constants at the top of the file.

Compatibility
Desktop/mouse behaviour is intentionally unchanged: the mouse drives the same pointer code paths, the hover highlight stays mouse-only, and the native double-click and right-click handlers are preserved.

Testing
Tested on desktop with a mouse and in a mobile browser with touch: select, move, resize, draw, double-tap-to-edit, long-press menu, reorder, and the new "+ Box" button all behave as expected.

(Context: this follows up the mobile issue I reported and the earlier attempt in dc2cc18 — that commit didn't convert the canvas interaction to pointer events, which is the part that actually unblocks touch.)