ui: comprehensive UI polish pass

- LoginPage/SetupPage: centered card layout with display font, styled fields and submit button
- Topbar: fix duplicate Settings nav item; add styled user-menu with username + logout button
- RunsView: format ISO timestamps to readable dates, add colored status badges (completed/failed/running)
- AdminView: view-tabs system, styled admin-table, admin-input, status-badge (active/disabled), btn-primary
- SettingsView: replace all inline styles with form-section/form-field/field-input/btn-primary/btn-danger
- CollectionsView: restructure create form with proper field labels and btn-primary
- TagsView: add Tags section heading with separator
- ContextRail: show visibility as human-readable label not raw number; fix assign-error class
- styles.css: add auth-loading, view-tabs, form utilities, btn variants, status badges,
  run-status pills, token-banner/row, checkbox-row, coll-create-form, tag-tree-header CSS

frontend/src/components/ContextRail.jsx

@@ -70,7 +70,7 @@ export default function ContextRail({ archiveId, selectedEntry, onTagFilterSet,
     ['Added',      formatTimestamp(detail.summary.archived_at)],
     ['Source',     detail.summary.source_kind],
     ['Type',       detail.summary.entity_kind],
-    ['Visibility', detail.summary.visibility],
+    ['Visibility', VIS_LABEL[detail.summary.visibility] ?? detail.summary.visibility],
     ['Root',       detail.structured_root_relpath],
   ] : []
 
@@ -158,7 +158,7 @@ export default function ContextRail({ archiveId, selectedEntry, onTagFilterSet,
               </div>
             )}
             {assignError && (
-              <p style={{ color: 'var(--accent)', fontSize: 13, margin: '0 0 8px' }}>{assignError}</p>
+              <p className="form-msg form-msg--err" style={{ margin: '0 0 8px' }}>{assignError}</p>
             )}
             <div className="tag-input-wrap">
               <span className="hash">/</span>