Fix: enable global interactivity so dialogs/selects actually work

Editing a meter (any admin dialog, select dropdown, snackbar, dark-mode toggle)
did nothing: pages declared @rendermode InteractiveServer individually, but
MainLayout — which hosts MudDialogProvider/MudPopoverProvider/MudSnackbarProvider
— was rendered by <Routes>, which was static. Inline MudDialogs and MudSelect
popovers render through those providers, so with the providers non-interactive no
dialog could ever open.

Set the render mode on <Routes> and <HeadOutlet> in App.razor (global
interactivity) and removed the now-redundant per-page @rendermode declarations
(they would otherwise throw "parent already has a render mode").

Why it slipped through: the integration render tests only issue a GET (static
prerender), which never exercises the SignalR circuit. Verified this fix with a
real headless-browser run (Playwright): the meter edit dialog opens and the
"Sub-meter of (upstream meters)" multi-select opens with options — proving the
layout providers are now interactive. 116 tests still green.

Claude-Session: https://claude.ai/code/session_01Lz2RqAsnQhetqWNoCDfexK

src/App/Components/Pages/Admin/Connectors.razor

@@ -1,5 +1,4 @@
 @page "/admin/connectors"
-@rendermode InteractiveServer
 @inject Microsoft.EntityFrameworkCore.IDbContextFactory<MeterVault.Infrastructure.Persistence.MeterVaultDbContext> DbFactory
 @inject MeterVault.Infrastructure.Ingestion.HaConnectionTester HaTester
 @inject ISnackbar Snackbar