fix: honor light theme in playground

[nblog]

Summary

• Soften the light theme palette from near-white to a cooler gray surface system.
• Make global sidebar branding theme-aware so LUMEN X remains readable in light mode.
• Replace hard-coded dark playground surfaces with semantic theme tokens across the playground shell, controls, cards, gallery, and related modals/drawers.

Test plan

• Ran sensitive-data scans from the LumenX publish workflow and reviewed existing placeholder/documentation hits.
• Ran git diff --check on the staged frontend theme files.
• Verified #/settings and #/playground in light mode with Playwright; confirmed html.light, cold-gray body background, readable heading, and readable LUMEN branding.
• frontend typecheck is still blocked by existing unrelated EnvConfig / MULERUN_CLI_LOGGED_IN type errors in EnvConfigDialog.tsx and SettingsPage.tsx.

[CLAassistant]

All committers have signed the CLA.