Welcome modal as left-docked banner with angled edge

[milanofthe]

• Replaces the centered welcome modal with a left-docked banner (~58vw, ~760px max) that slides in from the left and slides out when dismissed.
• Right edge is angled via clip-path; the diagonal is drawn as a non-scaling SVG stroke in the same muted border colour as the in-banner separator.
• Adds a tagline under the logo: Visual block-diagram editor for the PathSim simulation framework.
• Backdrop keeps the existing blur, fades alongside the slide.