Add nodal load visualization with Grillage overlay and toolbar control

[Soham-1304]

Summary

This PR adds nodal load visualization for Load Cases and Load Combinations across the plot views and integrates it into the plot toolbar through a dedicated Loads toggle.

Applied loads are computed from the analysis results and rendered as nodal load arrows with force labels in Grillage, SFD, BMD, and Deflection plots. The arrows are drawn above the y=0 line, with the arrowhead anchored at the node and the arrowtail extending upward for clearer visualization.

To support this, Grillage was made an explicit plot-side option alongside Fx, Vy, and Vx. The Loads toolbar toggle now automatically enables Grillage when needed, since load visualization depends on the grillage nodes being present.

The toolbar integration was then aligned with the existing managed toggle workflow, so the Loads button follows the same state handling as the rest of the plot controls.

Changes

• Added nodal load arrows and force labels for Load Cases and Load Combinations.
• Positioned load arrows above the node line for clearer visualization.
• Added Grillage as a plot-side checkbox option alongside Fx, Vy, and Vx.
• Enabled load visualization across Grillage, SFD, BMD, and Deflection plots.
• Added a dedicated Loads toolbar toggle to show and hide load visualization.
• Made the Loads toggle automatically enable Grillage when required.
• Refactored the toolbar integration to use the standard managed toggle workflow and removed redundant state-management code.

[Nidhikhare12]

@Soham-1304 some issues with the load in plots. Its visible on the nodes of plots rather than the nodes on the grillage. Also, doesnt seems to be updating for certain loads. Lets discuss tomorrow.