feat(chat): view context — tell the assistant what you're looking at

An eye next to the paperclip shares what the user has open with their next
message: the page, the folder being browsed, the file open in the viewer and
any highlighted passage (line numbers where a source view exists), plus which
entity a detail page is about. The bag is client-authored {label, value} pairs
in English — the backend only clamps (chars, never bytes), neutralizes the
harness tag and renders one <system-extra> block per message, deduped
consecutively so it appears exactly when the view changed. On by default,
per-device toggle, hover/tap to preview, a chip on every sent message;
docs/view-context.md for users, an updated harness.md clause for the model.

web/i18n/en.js

@@ -73,6 +73,16 @@ export default {
   'chat.new_session':   'New conversation',
   'chat.scroll_to_latest': 'Scroll to latest',
   'chat.security_group': 'Security group',
+  // The eye in the composer. These are labels a person reads, so they are
+  // translated — unlike the view-context sentences themselves, which are written
+  // for the model and stay English (see web/lib/view-context-routes.js).
+  'chat.view_context.on':        'Sharing what you have open — click to stop',
+  'chat.view_context.off':       'Not sharing what you have open — click to share',
+  'chat.view_context.title':     'Sent with your next message',
+  'chat.view_context.off_title': 'Not shared',
+  'chat.view_context.off_hint':  'The assistant is not told which page, folder or file you have open. Turn the eye on to share it.',
+  'chat.view_context.empty':     'Nothing to share from this page yet.',
+  'chat.view_context.chip':      'What you had open ({n})',
   'chat.collapse':      'Hide chat',
   'chat.close_tab':     'Close tab',
   'chat.new_tab':       'New chat',