v0.31.4: fix invisible light-surface btn-link + harmonize RFC breadcrumb action bar

`.btn-link` was a dark-header utility (white text on translucent white)
reused on light surfaces — the RFC breadcrumb action bar, PR diff toggle,
modals, discussion panel — where it rendered white-on-white and looked
like missing buttons (reported: "Metadata"/"Claim ownership"/"Invitations"
absent on a super-draft header). Root-cause fix:

- Base .btn-link is now a light-surface secondary button (white fill,
  hairline border, dark label); the dark-header "Sign out" keeps the
  translucent-on-dark treatment via an .app-header .btn-link scope.
- .breadcrumb-actions normalizes the toggle, filled CTAs, and secondary
  buttons to one height/radius/type as a single control group, and
  flex-wraps instead of clipping off the right edge.
- Diff-mode active toggle now reads as clearly selected (filled ink).

CSS-only; patch release, plain frontend rebuild applies it. No upgrade
steps. Driver session 0059.0.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "rfc-app-frontend",
   "private": true,
-  "version": "0.31.3",
+  "version": "0.31.4",
   "type": "module",
   "scripts": {
     "dev": "vite",