Block icons: Simulink-style icon rendering with toggle

[milanofthe]

• Adds an opt-in icon display mode for blocks. Each block class gets a Simulink-style icon (programmatic plot, KaTeX math, mono glyph, framed scope/spectrum, 3D wireframe surface, or hand-drawn SVG).
• Global toggle via I shortcut, per-node override in the context menu (Show as Icon / Show as Text), persisted in localStorage. Falls back to type text where no icon is registered.
• Pinned plot previews (Scope/Spectrum) now factor into Fit-View bounds, and connections automatically reroute whenever a block's measured size changes.