fix: address mobile UI review findings (#16)

- Hide desktop .card-section on mobile, add .m-search-input with same
  HTMX attrs for mobile course search (fixes horizontal overflow)
- Remove dead layoutCount var and .m-layouts-pill block in course-cards
- Remove dead 768px breakpoints from players.css (table hidden at 880px)
- Move .mobile-section-head inside else-block for empty state in both
  ratings-cards and course-cards (fixes section head showing on empty)
- Add tabindex, role=button, aria-expanded, onkeydown to .m-card and
  .m-course-card; toggle aria-expanded in JS toggle functions
- Fix data-history attribute to use <%=  (HTML-escaped) instead of <%-
- Convert var to const/let in all new/changed JS blocks

public/css/mobile.css

@@ -13,6 +13,7 @@
 .mobile-add-bar      { display: none; }
 .mobile-section-head { display: none; }
 .m-tab-pill          { display: none; }
+.m-search-input      { display: none; }
 
 /* ═══════════════════════════════════════════════════ */
 @media (max-width: 880px) {
@@ -24,6 +25,37 @@
     .add-bar          { display: none !important; }
     .footnote         { display: none !important; }
 
+    /* Hide desktop search card on mobile (mobile has .m-search-input instead) */
+    .card-section     { display: none; }
+
+    /* ── Mobile course search input ─────────────────── */
+
+    .m-search-input {
+        display: block;
+        width: 100%;
+        height: 38px;
+        padding: 0 12px;
+        background: var(--paper-2);
+        border: 1px solid var(--line-2);
+        border-radius: 10px;
+        font-family: var(--font-sans);
+        font-size: 14px;
+        color: var(--ink);
+        outline: none;
+        box-sizing: border-box;
+        transition: border-color 150ms ease, box-shadow 150ms ease;
+    }
+
+    .m-search-input::placeholder {
+        color: var(--ink-3);
+        opacity: 0.7;
+    }
+
+    .m-search-input:focus {
+        border-color: var(--accent);
+        box-shadow: 0 0 0 3px color-mix(in oklab, var(--accent) 14%, transparent);
+    }
+
     /* Hide desktop table but keep .table-card as wrapper */
     .table-card > #ratings-table table,
     #ratings-table table,