Let suggestion filtering narrow to a specific archetype or tribe

Filtering by category alone wasn't granular enough: a deck usually matches
several archetypes and tribes at once, so "show me Archetype suggestions"
still returns most of the list.

SynergyReason now carries the matched thing itself (`subject`) alongside the
human-readable `detail`, populated at all four match sites. The modal adds a
second chip row listing every subject present in the current suggestions,
sorted by how many cards each explains and labelled with that count, so the
most useful narrowing is obvious before clicking.

src/styles.css

@@ -2757,3 +2757,12 @@ tbody tr:hover {
   overflow-wrap: anywhere;
   min-width: 0;
 }
+
+/* Suggestion count inside a "narrow to" chip — how many suggestions that
+   specific tribe/keyword/archetype explains, so the most useful narrowing is
+   obvious before clicking. */
+.filter-chip-count {
+  opacity: 0.55;
+  font-variant-numeric: tabular-nums;
+  margin-left: 2px;
+}