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>

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.114.0"
+version = "0.114.1"
 edition = "2021"
 default-run = "mangalord"