Achievement badges + milestone credit bonuses

- Idempotent one-time credit bonus per milestone (payouts/firstBuyer/level2/level3): 10/25/50/100 cr
- Overview badges strip (unlockable medals) with canvas share-to-image (username overlaid)
- Newly-unlocked milestones toast the bonus on next dashboard load

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

public/assets/site.css

@@ -259,6 +259,18 @@ textarea{resize:vertical;font:inherit}
   #boBurger{display:block}
   .bo-content{padding:18px}
 }
+/* ── achievement badges ── */
+.badges{display:flex;gap:16px;flex-wrap:wrap;margin-top:12px}
+.badge-a{display:flex;flex-direction:column;align-items:center;gap:8px;width:120px;text-align:center}
+.badge-a .medal{width:76px;height:76px;border-radius:50%;display:grid;place-items:center;position:relative;
+  background:radial-gradient(circle at 50% 35%,rgba(67,232,195,.28),rgba(9,26,19,.9));
+  border:2px solid var(--mint);box-shadow:0 0 18px rgba(67,232,195,.3)}
+.badge-a.locked .medal{background:rgba(139,166,156,.08);border-color:var(--line-strong);box-shadow:none;filter:grayscale(1);opacity:.5}
+.badge-a .medal svg{width:38px;height:38px;stroke:var(--mint);fill:none;stroke-width:1.7;stroke-linecap:round;stroke-linejoin:round}
+.badge-a.locked .medal svg{stroke:var(--muted)}
+.badge-a .bl{font-size:12px;font-weight:700;line-height:1.2}
+.badge-a .bs{font-size:10.5px;color:var(--muted)}
+.badge-a .share{font-size:11px;color:var(--mint);cursor:pointer;background:none;border:0;padding:0;text-decoration:underline}
 /* ── "Your next move": milestone stepper card ── */
 .next-card{position:relative;overflow:hidden;border-color:rgba(67,232,195,.28)}
 .next-card::before{content:"";position:absolute;inset:0;pointer-events:none;background: