Add in-popup confirmation for unsaved settings

[aminought]

Summary

• show a reusable focus-layer confirmation inside the Web Panel and Sidebar settings popups before discarding unsaved changes
• use one field-level change/revert definition in each editor for both dirty detection and rollback
• preserve edits on Keep Editing and restore the original values on Discard, including shortcuts, zoom, pending favicon requests, and combined URL/CSS selector changes
• keep keyboard focus inside the confirmation and restore it when editing continues
• handle Firefox startup readiness without missing sidebar initialization

Closes #155

Validation

• npx.cmd eslint .
• npx.cmd prettier --check for all changed files
• node --check for all changed JavaScript modules
• git diff --check
• Firefox Stable 155.0.1 startup smoke test at commit a036bc2: 152 files verified, nested panel browser ready, and no FSS or fx-autoconfig errors captured

The focus-layer interaction was manually verified in Firefox Stable; the final review refactor was covered by static checks and a fresh startup smoke test.