Add Browse and Alarms dashboard tabs

Browse renders the Galaxy hierarchy tree from IGalaxyHierarchyCache:
expandable areas/objects with attribute name, data type and the
alarm/historized flags, plus a name/reference filter. Right-click or
double-click an attribute to add it to a subscription panel that polls
live value, quality and source timestamp every two seconds.

Alarms lists the worker's currently-active alarm set via
IAlarmRpcDispatcher, defaulting to unacknowledged Active alarms with
filters for acknowledged alarms, area, severity range and text. It is
read-only and warns when alarm auto-subscribe is disabled.

Both tabs read live MXAccess data through a new singleton
DashboardLiveDataService that owns one shared, lazily-opened gateway
session (one worker) for the whole dashboard, re-opened transparently
if it faults or its lease expires.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/GatewayDashboardDesign.md

@@ -254,6 +254,49 @@ Show aggregate event diagnostics:
 Do not display full tag values by default. If value display is later added, make
 it opt-in and redacted.
 
+### Browse page
+
+`/dashboard/browse` lets an operator explore the Galaxy tag hierarchy and watch
+live values. The tree is built in-process by `DashboardBrowseTreeBuilder` from
+`IGalaxyHierarchyCache.Current` — the same cache the Galaxy page reads — so a
+render costs no gRPC call and no SQL round-trip. Each node shows its child
+objects and, when expanded, its attributes with attribute name, data type
+(including array dimension), and the alarm / historized flags. Galaxy SQL
+carries no attribute description, so none is shown. A filter box switches the
+tree to a flat list of matching attributes.
+
+Right-clicking an attribute (or double-clicking it) adds it to the subscription
+panel. The panel shows each subscribed tag's live value, MXAccess data type,
+quality and source timestamp, refreshed every two seconds. The subscription
+panel is the explicit opt-in tag-value surface: it always shows values
+regardless of `Dashboard:ShowTagValues`, which continues to govern only the
+diagnostic session/worker views.
+
+### Alarms page
+
+`/dashboard/alarms` lists the alarms the dashboard session's worker currently
+reports as Active or ActiveAcked, refreshed every three seconds. It defaults to
+showing unacknowledged `Active` alarms; filters add acknowledged alarms and
+narrow by area, severity range, and a reference/source/description text search.
+Cleared alarms are not retained — the gateway holds no alarm-history store, so
+the page reflects only the live active set. The page is read-only; it does not
+acknowledge alarms. If `MxGateway:Alarms:Enabled` is false the session is never
+subscribed to an alarm provider, and the page says so instead of showing an
+empty list with no explanation.
+
+### Live data source
+
+Both the Browse subscription panel and the Alarms page read live MXAccess data
+through `IDashboardLiveDataService` (`DashboardLiveDataService`). It owns one
+shared gateway session for the whole dashboard, opened lazily on first use via
+`ISessionManager` and re-opened transparently when it faults or its lease
+expires. One session means one worker process backs every dashboard circuit;
+all access is serialised so the worker sees one in-flight command at a time.
+Tag reads go through `GatewaySession.SubscribeBulkAsync` / `ReadBulkAsync`;
+alarm queries go through `IAlarmRpcDispatcher`. Alarm subscription is the
+gateway's existing auto-subscribe-on-open hook, so the dashboard session is
+alarm-subscribed only when `MxGateway:Alarms:Enabled` is set.
+
 ### API keys page
 
 `/dashboard/apikeys` lists the gateway's API keys and, for authorized