fix: add persisted theme toggle

[tristanwork]

Summary

• add a reusable theme toggle controller for settings and navigation UI
• persist the selected light/dark theme in localStorage-compatible storage
• restore the saved preference on startup
• update toggle label and pressed state for accessible controls

Testing

• npm test

Fixes #11
Fixes #19

[tristanwork]

Local verification passes on this branch:

npm test
# parser tests: 6 passed
# theme preference tests: 11 passed

The GitHub Actions check is currently action_required with no jobs started, so it appears to need maintainer approval to run for this fork PR.