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.

docs/view-context.md

@@ -12,7 +12,7 @@ Exactly one snapshot per message, covering whatever applies at that moment:
 - **A highlighted passage** — if the user selected text in the viewer, the selected text itself, with its line numbers when they are looking at the source (a plain-text file, or the editor view of a Markdown file).
 - **Which thing a detail page is about** — which project (and which of its tabs), which member, connector, plugin, conversation, tool call or LLM request; also the active section in Tasks or Models, the open agent in Background agents, and a search typed in the Marketplace.
 
-Hover the eye (or tap it, on a touch screen) to read exactly what would be sent with the next message. Every sent message shows a small chip with what it carried, which can be opened to read the actual values.
+Hover the eye (or tap it, on a touch screen) to read exactly what would be sent with the next message. Every sent message keeps a small eye in its corner; hovering it (or tapping it, on a touch screen) shows the actual values that message carried.
 
 ## Control and privacy