fix(view-context): a corner mark on the bubble, not a row under it

The sent-message proof was a collapsed <details> under the user bubble: a
row of height in every bubble that carried view context, for something that
is evidence about the message rather than part of it. It also rendered a
blank line, since the bubble is white-space: pre-wrap and the template left
a newline before the element.

Now a faint eye in the bubble's bottom-right corner, revealing the pairs on
hover (on focus for keyboard and touch). Deliberately not expandable, so
scrolling back through a conversation cannot grow a second layout, and the
popover opens downwards rather than over the message it belongs to.

The i18n key stays alive as the mark's aria-label; docs and changelog wording
updated to match.

web/components/copilot-render.js

@@ -591,18 +591,22 @@ export function renderViewContextPill(host) {
  * The proof, in the sent bubble: what this message actually carried. Rendered
  * from the server's echo (and, after a reload, from the REST history), so it
  * shows the sanitized pairs the model was given — never the browser's intent.
+ *
+ * A mark in the bubble's corner, not a row under the message: this is evidence
+ * about a message, not part of it, and a collapsed row still cost a line of
+ * height in every bubble that carried one. Deliberately **not** expandable —
+ * the pairs show on hover (on focus for keyboard and touch) and there is no
+ * state to leave open, so a conversation scrolled back through never grows a
+ * second layout.
+ *
+ * Written on one line on purpose: the bubble renders `white-space: pre-wrap`, so
+ * the newline and indent a formatted template would leave before the element are
+ * a text node the bubble actually shows — a blank line under the message.
  */
-function renderViewContextChip(host, msg) {
+function renderViewContextMark(host, msg) {
   const items = msg.view_context;
   if (!items?.length) return nothing;
-  return html`
-    <details class="view-ctx-chip">
-      <summary>
-        <i class="bi bi-eye"></i>
-        <span>${t('chat.view_context.chip', { n: items.length })}</span>
-      </summary>
-      ${renderViewContextItems(items)}
-    </details>`;
+  return html`<div class="view-ctx-mark" tabindex="0" aria-label=${t('chat.view_context.chip', { n: items.length })}><i class="bi bi-eye"></i><div class="view-ctx-pop">${renderViewContextItems(items)}</div></div>`;
 }
 
 /**
@@ -625,7 +629,7 @@ export function renderMsg(host, msg) {
   try {
     switch (msg.kind) {
       case 'user':
-        return html`<div class="copilot-msg user ${msg.failed ? 'copilot-msg--failed' : ''}" style="white-space:pre-wrap">${msg.failed ? failedBadge() : nothing}${msg.content}${renderAttachmentChips(host, msg.attachments)}${renderViewContextChip(host, msg)}</div>`;
+        return html`<div class="copilot-msg user ${msg.view_context?.length ? 'copilot-msg--view-ctx' : ''} ${msg.failed ? 'copilot-msg--failed' : ''}" style="white-space:pre-wrap">${msg.failed ? failedBadge() : nothing}${msg.content}${renderAttachmentChips(host, msg.attachments)}${renderViewContextMark(host, msg)}</div>`;
       case 'thinking':
         return html`
           <div class="copilot-msg assistant copilot-markdown ${msg.failed ? 'copilot-msg--failed' : ''}">