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/css/copilot-messages.css

@@ -889,33 +889,72 @@
   line-height: 1.35;
 }
 
-/* The chip inside a sent user bubble: collapsed by default, same muted weight as
-   the reasoning block — it is evidence, not content. */
-.view-ctx-chip {
-  margin-top: 0.4rem;
-  font-size: 0.75rem;
-  white-space: normal;
+/* The mark in a sent user bubble's corner: it is evidence about the message, not
+   part of it, so it costs the bubble a corner rather than a row of its own. Kept
+   faint on purpose — it should be findable when looked for and invisible when
+   not. Hover (focus, on touch and for the keyboard) reveals the pairs; there is
+   no expanded state, by design. */
+.copilot-msg.user.copilot-msg--view-ctx {
+  position: relative;
+  /* Room for the mark, so a last line can never run under it. */
+  padding-bottom: 1.35rem;
 }
 
-.view-ctx-chip > summary {
+.view-ctx-mark {
+  position: absolute;
+  right: 0.5rem;
+  bottom: 0.25rem;
   display: inline-flex;
   align-items: center;
-  gap: 0.3rem;
-  padding: 0.2rem 0.45rem;
-  border: 1px solid var(--toolbar-border);
-  border-radius: 0.5rem;
-  color: var(--placeholder-color);
-  cursor: pointer;
-  list-style: none;
-  line-height: 1.2;
+  font-size: 0.8rem;
+  line-height: 1;
+  color: inherit;
+  opacity: 0.4;
+  cursor: default;
+  transition: opacity 0.12s;
 }
 
-.view-ctx-chip > summary::-webkit-details-marker { display: none; }
-.view-ctx-chip > summary:hover { border-color: var(--accent); color: var(--accent); }
+.view-ctx-mark:hover,
+.view-ctx-mark:focus { opacity: 0.9; outline: none; }
+.view-ctx-mark:focus-visible { opacity: 0.9; outline: 2px solid currentColor; outline-offset: 2px; border-radius: 0.2rem; }
 
-.view-ctx-chip .view-ctx-items {
-  margin-top: 0.4rem;
-  padding-left: 0.15rem;
+/* Below the bubble, right-aligned with it: above would cover the message the
+   mark belongs to. */
+.view-ctx-pop {
+  position: absolute;
+  top: calc(100% + 6px);
+  right: 0;
+  z-index: 20;
+  display: block;
+  width: max-content;
+  max-width: min(26rem, 70vw);
+  max-height: 16rem;
+  overflow-y: auto;
+  padding: 0.5rem 0.65rem;
+  border: 1px solid var(--toolbar-border);
+  border-radius: 0.5rem;
+  background: var(--msg-assistant-bg);
+  color: var(--msg-assistant-text);
+  box-shadow: 0 4px 14px rgba(0, 0, 0, 0.12);
+  font-size: 0.75rem;
+  /* The bubble is pre-wrap; the popover formats its own values. */
+  white-space: normal;
+  text-align: left;
+  opacity: 0;
+  visibility: hidden;
+  pointer-events: none;
+  transition: opacity 0.12s;
+}
+
+/* Interactive only while shown: `:hover` applies to ancestors of the hovered
+   element, so letting the pointer onto the popover keeps the mark hovered — which
+   is what makes a long, scrollable value readable. Hidden, it must not eat a
+   click on the bubble underneath. */
+.view-ctx-mark:hover .view-ctx-pop,
+.view-ctx-mark:focus .view-ctx-pop {
+  opacity: 1;
+  visibility: visible;
+  pointer-events: auto;
 }
 
 /* ── Tool card "view details" (eye) ────────────────────────────────────────── */