[0.4.0] Pfosten and foundation calculator launch

Release: 0.4.0

Added

• Added Pfostenrechner (/pfosten-berechnen) with Eurocode 5 buckling verification for timber columns
• Added wood class support (C14-C40, GL24h-GL32h) with automatic compression strength checks
• Added real-time canvas visualization for load distribution and column geometry
• Added Fundamentrechner (/fundament-berechnen) with soil bearing capacity checks
• Added seven soil types (Kies, Sand, Ton/Lehm, and more) with mapped bearing values
• Added concrete class selection (C12/15 to C50/60) and reinforcement type configuration
• Added interactive foundation canvas for geometry and stress visualization
• Added learning content tabs for both new calculators

Changed

• Added mega navigation menu for faster switching between calculators
• Added results overlay with interactive tooltips and click handling

Notes

• Foundation outputs are sizing guidance for early feasibility checks and iteration, not engineering approval