Sponsor to downline messaging: lineage view, daily broadcast (email+inbox), login modal

- Downline resolver (3 levels); email visible for directs only, username+ID deeper
- Broadcast composer (rich editor) to directs or full downline, 1/day cap, sends email + on-site inbox
- Upline messages inbox in My line; unmissable sign-in modal for unread, ack marks read
- messages.js dual-mode store; sponsor_messages table; sanitizeRich reused

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

public/assets/site.css

@@ -341,6 +341,17 @@ textarea{resize:vertical;font:inherit}
 .wall-card{background:var(--panel);border:1px solid var(--line);border-radius:var(--radius);padding:16px;text-align:center}
 .wall-card img{max-width:100%;border-radius:10px;border:1px solid var(--line-strong)}
 .wall-pos{font-family:var(--mono);font-size:11px;color:var(--muted);text-transform:uppercase;letter-spacing:.08em;margin-bottom:8px}
+/* ── modal (sponsor message) ── */
+.modal-back{position:fixed;inset:0;z-index:100;background:rgba(2,6,5,.72);display:flex;align-items:center;justify-content:center;padding:20px}
+.modal-card{background:var(--panel-solid);border:1px solid var(--line-strong);border-radius:var(--radius);
+  padding:24px;max-width:520px;width:100%;box-shadow:0 20px 60px rgba(0,0,0,.5)}
+/* ── downline lineage list ── */
+.lin-lvl{margin:10px 0}
+.lin-lvl>.cap{font-size:11px;text-transform:uppercase;letter-spacing:.08em;color:var(--muted);margin-bottom:6px}
+.lin-row{display:flex;gap:10px;align-items:baseline;padding:6px 0;border-bottom:1px solid var(--line);flex-wrap:wrap}
+.lin-row .nm{font-weight:700;min-width:120px}
+.lin-row .em{font-size:12px;color:var(--mint);overflow-wrap:anywhere}
+.lin-row .id{font-family:var(--mono);font-size:11px;color:var(--muted)}
 /* ── solo composer: toolbar + contenteditable editor ── */
 .ed-bar{display:flex;gap:6px;flex-wrap:wrap;margin-bottom:8px}
 .ed-bar button{background:var(--panel);color:var(--ink);border:1px solid var(--line-strong);border-radius:8px;