terminal: build the ws URL like the app's own downlinks; show the attempted URL on failure

The served bundle's wsUrl is provably correct, yet one report showed
ws://host/ws//host/sidebar/ws/terminal (a /ws/ prefix no version ever
emitted). Mirror the app's own downlink construction (new URL(path,
location.origin) + protocol swap — the construction the chat's working
event streams use), and surface the exact attempted URL in the failure
banner plus a console.error, so a recurrence identifies the source
(stale page vs environment rewrite) immediately.

src/client/sidebar.module.css

@@ -662,6 +662,7 @@
   flex: none;
   display: flex;
   align-items: center;
+  flex-wrap: wrap;
   gap: 8px;
   padding: 3px 10px;
   font: var(--dsw-font-xxxs-11);
@@ -669,6 +670,14 @@
   background: var(--dsw-alias-state-warn-tertiary);
 }
 
+/* The attempted ws URL, shown when the connection failed (diagnosis). */
+.terminalBannerUrl {
+  flex-basis: 100%;
+  font-family: ui-monospace, SFMono-Regular, Menlo, monospace;
+  word-break: break-all;
+  opacity: 0.85;
+}
+
 .boundaryError {
   position: fixed;
   right: 0;