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/app.js

@@ -38,6 +38,9 @@ import { LoginPage }        from './components/login-page.js';
 // Register the global `openFile(path)` / `openToolDetail(id)` helpers.
 import './lib/open-file.js';
 import './lib/open-tool.js';
+// The view-context store keeps its own `route` slice in step with navigation,
+// so it has to be loaded from boot — not lazily by whoever reads it first.
+import './lib/view-context.js';
 import { initI18n } from './lib/i18n.js';
 import { installSessionExpiryWatch } from './lib/session-expiry.js';
 import { installSessionRelogin }     from './components/session-relogin.js';