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>

config.lua

@@ -6,4 +6,5 @@ return {
     threshold = 4,         -- total synergy score needed to recommend a card
     learning_mode = false, -- hide verdicts (pulse, "Strong pick!"), keep reasons
     discovered = {},       -- famous pairs the player has fielded (pair keys)
+    themes_found = {},     -- synergy themes the player has assembled (tags)
 }