fix(scripts): guard persist_design_system against None project_name (#159)

[ia-abatista]

What & why

persist_design_system crashes with AttributeError: 'NoneType' object has no attribute 'lower' (#159) when the design_system dict carries an explicit project_name of None:

project_name = design_system.get("project_name", "default")  # returns None when key is present-but-None
project_slug = project_name.lower().replace(' ', '-')        # 💥

dict.get(key, default) only substitutes the default for a missing key, not a present-but-None value — and generate(query, project_name=None) is the documented default, so a None reaches here easily.

Fix

Coalesce falsy values (None / "" / missing) to "default" before slugifying:

project_name = design_system.get("project_name") or "default"

Applied to both the source of truth (src/ui-ux-pro-max/scripts/) and the bundled cli/assets/ copy.

Verification

project_name=None          -> OK (no crash)
project_name=''            -> OK (no crash)
project_name='My Cool App' -> OK (no crash)
missing key                -> OK (no crash)

npm run check:assets passes (both copies in sync).

Scope

#159 also cited the core.py tokenize length filter ("search blindness"); that's already resolved on main (len(w) >= 2), so this PR targets only the remaining crash.

Supersedes #160 (same root cause; this version coalesces "" as well and keeps the two script copies in sync). Credit to @YangKuoshih for the original diagnosis and fix.

Closes #159