UI improvements and settings Improvements

• Ignored Directories panel overhaul: directory browser, add/clear controls, descriptive labels, confirmation before purging indexed docs on add, confirmation before removing an entry
• New "Additional Scan Directories" capability — scan multiple directories beyond the primary docPath, backed by scan_dirs.txt and /api/scan-dirs, with the exact rescan CLI command shown when adding a directory
• Consolidated all directory-management UI (docPath, additional scan dirs, workDir) into the General settings panel — no more jumping between sections for one workflow
• docPath and workDir "Browse…" buttons now live-sync the chosen path directly into the field (matching Ignored Directories UX), removing the old "select this" step
• New settings screenshot reflecting the redesigned UI
• Docs updated: README, DECISIONS, project_status, INSTALL