Restore dashboard @page "/dashboard/X" directives — Server-020 reversal

Server-020 incorrectly removed the duplicate `@page "/dashboard/X"` directive
from each dashboard Razor page on the assumption that `MapGroup("/dashboard")`
would prepend the prefix to Blazor SSR route matching. It does not — Blazor's
`@page` template matcher operates on the full URL path, not relative to a
MapGroup. The removal left the dashboard returning HTTP 500 with
"Unable to find the provided template '/dashboard/'" from
RouteTableFactory.CreateEntry on every page.

Restored the eight `@page "/dashboard/X"` directives. The accompanying
regression test still passes (it asserts the genuinely-double-prefixed shape
`/dashboard/dashboard/X` never appears — it never did, since the original
duplicates were `"/"` + `"/dashboard/"`, not `"/dashboard/"` repeated). XML
doc on the test rewritten to record what was actually wrong with Server-020.

Verified: gateway redeploy + `GET http://localhost:5130/dashboard` returns
HTTP 200 7.3 KB; `/dashboard/sessions` also 200.

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

src/MxGateway.Server/Dashboard/Components/Pages/ApiKeysPage.razor

@@ -1,4 +1,5 @@
 @page "/apikeys"
+@page "/dashboard/apikeys"
 @inherits DashboardPageBase
 @inject AuthenticationStateProvider AuthenticationStateProvider
 @inject IDashboardApiKeyManagementService ApiKeyManagementService

src/MxGateway.Server/Dashboard/Components/Pages/DashboardHome.razor

@@ -1,4 +1,5 @@
 @page "/"
+@page "/dashboard/"
 @inherits DashboardPageBase
 
 <PageTitle>MXAccess Gateway Dashboard</PageTitle>

src/MxGateway.Server/Dashboard/Components/Pages/EventsPage.razor

@@ -1,4 +1,5 @@
 @page "/events"
+@page "/dashboard/events"
 @inherits DashboardPageBase
 
 <PageTitle>Dashboard Events</PageTitle>

src/MxGateway.Server/Dashboard/Components/Pages/GalaxyPage.razor

@@ -1,4 +1,5 @@
 @page "/galaxy"
+@page "/dashboard/galaxy"
 @inherits DashboardPageBase
 
 <PageTitle>Dashboard Galaxy</PageTitle>

src/MxGateway.Server/Dashboard/Components/Pages/SessionDetailsPage.razor

@@ -1,4 +1,5 @@
 @page "/sessions/{SessionId}"
+@page "/dashboard/sessions/{SessionId}"
 @inherits DashboardPageBase
 
 <PageTitle>Dashboard Session</PageTitle>

src/MxGateway.Server/Dashboard/Components/Pages/SessionsPage.razor

@@ -1,4 +1,5 @@
 @page "/sessions"
+@page "/dashboard/sessions"
 @inherits DashboardPageBase
 
 <PageTitle>Dashboard Sessions</PageTitle>

src/MxGateway.Server/Dashboard/Components/Pages/SettingsPage.razor

@@ -1,4 +1,5 @@
 @page "/settings"
+@page "/dashboard/settings"
 @inherits DashboardPageBase
 
 <PageTitle>Dashboard Settings</PageTitle>

src/MxGateway.Server/Dashboard/Components/Pages/WorkersPage.razor

@@ -1,4 +1,5 @@
 @page "/workers"
+@page "/dashboard/workers"
 @inherits DashboardPageBase
 
 <PageTitle>Dashboard Workers</PageTitle>