refactor: replace inline dropdown display toggling with reusable floating positioning helpers

Switch download target dropdowns from absolute-positioned elements clipped by
scroll containers to body-ported fixed-position lists managed by
downloadTargetMethods. Adds showDropdownList, hideDropdownList,
portalDropdownList, positionFloatingDropdownList, and related helpers so the
UI can keep dropdowns visible and correctly placed near their anchors.

web/css/resolver-main.css

@@ -5642,12 +5642,16 @@
     }
 }
 .mr-download-target-list {
+    box-sizing: border-box;
     position: absolute;
     top: calc(100% + 4px);
     left: 0;
-    right: 0;
+    right: auto;
+    width: 100%;
+    min-width: 100%;
     max-height: 220px;
-    overflow: auto;
+    overflow-x: hidden;
+    overflow-y: auto;
     display: none;
     z-index: var(--mr-z-notification);
     background: linear-gradient(180deg, var(--mr-panel-bg-strong) 0%, var(--mr-card-bg) 100%);
@@ -5669,11 +5673,16 @@
     z-index: var(--mr-z-context-menu);
 }
 .mr-download-target-option {
+    box-sizing: border-box;
+    max-width: 100%;
     padding: 8px 10px;
     cursor: pointer;
     border-bottom: 1px solid rgba(255,255,255,0.04);
     color: var(--mr-text);
     font-size: 12px;
+    overflow: hidden;
+    text-overflow: ellipsis;
+    white-space: nowrap;
 }
 .mr-download-target-option:last-child {
     border-bottom: none;