fix: modal dialogs now visible — z-index, sizing, overflow fixes

Modals (AllocationModal, BillingRuleModal, RefundModal) were dimming
the background but the dialog content was invisible because:
- overflow:hidden on the app container created a stacking context
  that trapped position:fixed elements
- Modal z-index was 1000, bumped to 9999
- Removed overflow:hidden from app container and html/body
- Added explicit width/height (100vw/100vh) to modal overlay
- Added min-width, max-width, max-height, overflow-y to modal body
- Modal content now scrolls independently if it exceeds 80vh

Author: Sqoia Dev Agent

src/slurmledger.css

@@ -2,7 +2,6 @@ html, body, #root {
   height: 100%;
   margin: 0;
   padding: 0;
-  overflow: hidden;
 }
 
 body {
@@ -286,19 +285,30 @@ nav button:hover {
 /* Modal overlay for refunds and other dialogs */
 .modal-overlay {
   position: fixed;
-  inset: 0;
+  top: 0;
+  left: 0;
+  right: 0;
+  bottom: 0;
+  width: 100vw;
+  height: 100vh;
   background: rgba(0, 0, 0, 0.5);
   display: flex;
   align-items: center;
   justify-content: center;
-  z-index: 1000;
+  z-index: 9999;
 }
 
 .modal {
   background: #fff;
-  border-radius: 6px;
-  padding: 1.5em;
-  box-shadow: 0 8px 32px rgba(0, 0, 0, 0.2);
+  border-radius: 8px;
+  padding: 1.5em 2em;
+  box-shadow: 0 8px 32px rgba(0, 0, 0, 0.3);
+  min-width: 400px;
+  max-width: 600px;
+  max-height: 80vh;
+  overflow-y: auto;
+  z-index: 10000;
+  position: relative;
 }
 
 .modal h3 {

src/slurmledger.js

@@ -6241,8 +6241,7 @@ function App() {
       style: {
         display: 'flex',
         flexDirection: 'column',
-        height: '100vh',
-        overflow: 'hidden'
+        height: '100vh'
       }
     },