Redesign settings extensions panel UX

[itsDNNS]

Summary

• Group extensions into Features and Modules sections with labeled dividers for scannability
• Enabled toggles get a full-bleed background highlight with left accent (CSS :has(), zero JS)
• Rewrite community registry card rendering from innerHTML + inline styles to DOM API + CSS classes
• Two-step confirmation for install/uninstall (first click shows "Confirm?", auto-resets after 3s)
• Loading spinner while registry fetches, proper empty state with icon, error toast on failure
• Change "Browse" button to icon-only refresh (registry auto-loads on panel visit)
• Fix toggle slider border-radius to pill shape
• Add in-flight guard to prevent concurrent registry fetches
• Add --card-padding token, .registry-status-warn/.registry-status-good color classes
• i18n: 7 new keys per language (EN/DE/FR/ES), all validated in sync

Test plan

• Open Settings > Extensions, verify Features/Modules sections render with dividers
• Toggle extensions on/off, confirm active state highlight appears/disappears
• Check community registry loads automatically with spinner, then shows cards or empty state
• Click Install on a community module, verify "Confirm?" appears, auto-resets after 3s
• Confirm install, verify restart banner and toast appear
• Click Uninstall, verify same two-step pattern
• Switch language to DE/FR/ES, verify all new strings are translated
• Rapidly switch away from and back to Extensions tab, verify no duplicate fetches
• Disconnect network, verify error toast appears on registry fetch failure