Add theme-level discovery

A synergy theme counts as explored the first time two different
fielded jokers connect it (one gives the tag, one wants it; cluster
members carry both sides, so any two qualify). Explored themes
persist in config.themes_found, badge their catalog page with
"explored!", and feed a per-tab progress line. One toast per
emplacement, with famous pairs outranking themes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -16,6 +16,10 @@ recommends joker combos while you play:
   "New combo!" toast fires and the pair is permanently inked into the
   catalog — until then its cards sit face-down with a "???" caption, so
   filling in the Combos tab becomes its own collection hunt.
+- **Theme discovery**: assembling any synergy theme for the first time —
+  one joker that provides it plus another that wants it — fires a
+  "Theme explored!" toast, badges that theme's catalog page, and counts
+  toward a per-tab progress line ("3 of 10 themes explored").
 - **Hover a joker you own** to see its active combos with the rest of your
   lineup.
 - **Browse the synergy catalog** any time: a purple **Combos** button in the