fix: define missing --font-md and --success-soft-bg tokens

Both were referenced by components (RecommendationShelf, the manga detail
page) but never declared, so var() fell through to inherited/fallback
values. Adds --font-md (1rem, on the xs/sm/md/lg scale) and
--success-soft-bg in both the light and dark theme blocks.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/styles/tokens.css

@@ -16,6 +16,7 @@
     --danger: #b00020;
     --danger-soft-bg: #fff5f5;
     --success: #0a7d2c;
+    --success-soft-bg: #eaf7ee;
     --warning-soft-bg: #fff5d6;
     --warning-border: #d6a800;
     --focus-ring: #2563eb;
@@ -28,6 +29,9 @@
 
     --font-xs: 0.75rem;
     --font-sm: 0.875rem;
+    /* Alias of --font-base at the body size; named on the xs/sm/md/lg/xl
+       scale that some components reach for. */
+    --font-md: 1rem;
     --font-base: 1rem;
     --font-lg: 1.125rem;
     --font-xl: 1.5rem;
@@ -117,6 +121,7 @@
     --danger: #f87171;
     --danger-soft-bg: #3a1620;
     --success: #4ade80;
+    --success-soft-bg: #12301c;
     --warning-soft-bg: #3a2e10;
     --warning-border: #a37800;
     --focus-ring: #60a5fa;