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/components/sidebar.js

@@ -1,6 +1,9 @@
 import { html, nothing } from 'lit';
 import { LightElement } from '../lib/base.js';
 import { t, I18nMixin } from '../lib/i18n.js';
+// Shared with the view-context store: one reading of the hash, so the page the
+// assistant is told about is always the page the menu highlights.
+import { pageFromHash } from '../lib/routes.js';
 
 
 // ── Navigation model ──────────────────────────────────────────────────────────
@@ -237,22 +240,7 @@ export class AppSidebar extends I18nMixin(LightElement) {
   }
 
   _pageFromHash() {
-    const hash = location.hash.slice(1);
-    if (!hash) return 'home';
-    // Segment ends at the first `/` (e.g. `#session/123`) or `?` (e.g. `#file_viewer?path=...`).
-    const match = hash.match(/^([^/?]+)/);
-    const segment = match ? match[1] : '';
-    // Plugin pages: `#plugin/<plugin_id>/<page_id>` — the route is accepted by
-    // shape (deep links must survive the async `/api/plugins/pages` load); the
-    // host reports an error if the page turns out not to exist for this user.
-    if (segment === 'plugin') {
-      const m = hash.match(/^plugin\/([^/?]+)\/([^/?]+)/);
-      return m ? `plugin/${m[1]}/${m[2]}` : 'home';
-    }
-    // `connector` (singular) is the per-connector detail page, `connectors` the list.
-    // `plugin-catalog` is the pre-merge hash of what is now `#plugins`.
-    const page = segment === 'plugin-catalog' ? 'plugins' : segment;
-    return ['inbox', 'dashboard', 'tasks', 'projects', 'files', 'models', 'providers', 'approval', 'agents', 'users', 'roles', 'shared-folders', 'connectors', 'connector', 'plugins', 'plugin-detail', 'marketplace', 'profile', 'config', 'llm-requests', 'session', 'system-agents', 'file_viewer', 'tool_detail'].includes(page) ? page : 'home';
+    return pageFromHash();
   }
 
   _tasksSectionFromHash() {