fix(custom-model): bound the model-picker dialog's height and make its list scroll

The dialog had no max-height at all, so an endpoint with many discovered
models grew it past the viewport with nothing to scroll — reported live as
both "takes up the full page" and "the list is truncated", which turn out
to be the same bug. Gives #customModelPickModal .modal-content the same
bounded-height + scrollable-body shape cronModal's .modal-lg already uses
(max-height + flex column on the content, overflow-y:auto + flex:1 on the
body), scoped by id rather than folded into the shared .modal-sm class
three other modals already use for short, fixed content.

max-height: min(70vh, 520px) scales with the viewport (a phone gets 70% of
its height; a 4K display never gets a needlessly tall dialog) rather than
committing to one fixed pixel value that would be wrong at either end.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RqZeHrRS6DYcGcGX2p9EwG

src/web/public/index.html

@@ -915,7 +915,7 @@
          selectCustomModelEntry() in session-ui.js, which skips straight to launch otherwise. -->
     <div class="modal" id="customModelPickModal">
       <div class="modal-backdrop" onclick="app.closeCustomModelPickModal()"></div>
-      <div class="modal-content" style="max-width: 380px;">
+      <div class="modal-content modal-sm">
         <div class="modal-header">
           <h3 id="customModelPickTitle">Choose a model</h3>
           <button class="modal-close" onclick="app.closeCustomModelPickModal()" aria-label="Close model picker">&times;</button>