Add dark mode toggle to catalog UI

[rajnavakoti]

Feature
Add a dark mode toggle to the catalog UI. The CSS design token system (RGB variables) is already in place, making this relatively low effort.

Scope
Add toggle button in the sidebar or header
Define dark mode color values for all CSS custom properties
Persist preference in localStorage
Respect system preference (prefers-color-scheme) as default
Ensure ReactFlow graphs, diagrams, and all components work in dark mode

---

Suggested implementation order:

Priority	Issues	Why first
P0	#1, #2, #3	IP cleanup — must do before wider visibility
P1	#4, #5, #11	Bug fixes + markdown rendering (core UX)
P2	#6, #7, #8, #9, #10, #17, #18	Polish + essential tooling
P3	#12, #13, #14, #20	Domain features + linting
P4	#15, #16, #19, #21, #22-28	Lower priority features, AI, docs

Want me to create these directly as GitHub issues using gh issue create, or do you want to review and adjust first?

[rajnavakoti]

Duplicate of #29. Deferring dark mode to focus on core priorities.