Keep modal OK button in view when message content overflows

The save-failure dialog now renders multi-section help text (headers,
list, link, several paragraphs), which exceeds the existing
max-height: 365px on .popup-modal.prompt. The message div was
overflowing past the modal box and pushing the OK button below the
visible area.

Restructure .popup-modal.prompt as a flex column:

- max-height: 80vh (more room when needed, still capped)
- Content area (#message) gets flex: 1 1 auto + overflow-y: auto, so
  long content scrolls inside the box.
- Buttons get flex: 0 0 auto, so they stay pinned at the bottom of
  the modal regardless of content size.
- The display: flex on .popup-modal.prompt.is--visible has three-class
  specificity, beating the existing .popup-modal.is--visible
  display: block rule without an !important.

Separately, cap the message dialog at max-width min(480px, 90vw) via
attribute selector so multi-paragraph prose wraps at a comfortable
reading measure (~60-65ch) rather than stretching across the full
viewport on wide monitors. Other prompt dialogs (unsaved, upload-type,
connection-type, ok-cancel) are short and unaffected.

Author: makermelissa-ai-assistant

sass/layout/_layout.scss

@@ -318,7 +318,32 @@
     display: none;
 
     &.prompt {
-        max-height: 365px;
+        max-height: 80vh;
+        // Flex column so the buttons stay pinned to the bottom and the
+        // content area can scroll when a dialog has rich/multi-section
+        // markup that overflows the modal height. Three-class selector
+        // beats `.popup-modal.is--visible` so the `display: flex`
+        // wins on specificity.
+        &.is--visible {
+            display: flex;
+            flex-direction: column;
+        }
+        #message {
+            overflow-y: auto;
+            flex: 1 1 auto;
+            min-height: 0;
+        }
+        .buttons {
+            flex: 0 0 auto;
+        }
+    }
+
+    // The message dialog often shows multi-section explanatory prose
+    // (e.g. the save-failure / read-only-filesystem help). Cap the
+    // width so paragraphs wrap at a comfortable reading measure rather
+    // than stretching the full viewport on a wide monitor.
+    &[data-popup-modal="message"] {
+        max-width: min(480px, 90vw);
     }
 
     &.shadow {