Fix ValueError in build_notebooks.py for relative path handling

[Copilot]

The build script failed when processing notebooks due to incorrect path handling. Path.rglob() returns relative paths, but the code unconditionally called .relative_to(Path.cwd()) on them, causing a ValueError.

Changes:

• Added path type check before conversion: only call .relative_to() on absolute paths
• Relative paths are now used as-is

# Before
rel = nb.relative_to(Path.cwd())  # Fails on relative paths

# After  
if nb.is_absolute():
    rel = nb.relative_to(Path.cwd())
else:
    rel = nb  # Already relative

This allows the script to execute all notebooks through to completion, with graceful error handling for notebooks with missing dependencies.

Original prompt

Make sure these errors are fixed and they can go all the way through to the very end

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[solveforceapp]

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