feat(browser): loopback allowlist with same-origin for local dev servers

Add a side-card preference 'browserAllowedLoopback' (comma-separated
host or host:port entries) that lets the sidebar browser navigate to
explicitly trusted loopback addresses. Bare hosts (localhost,
127.0.0.1) match every port; host:port entries match exactly.

Allowlisted local addresses additionally get the 'allow-same-origin'
sandbox token in the iframe: local dev servers (Vite etc.) need a real
origin for their module/HMR/fetch pipeline and would otherwise render
blank in the opaque-origin sandbox. The page stays cross-origin to the
GUI and to every other site.

The server-side browser.probe route mirrors the same allowlist so
probed local servers are not rejected as 'local addresses are not
probed'.

src/config.ts

@@ -95,6 +95,7 @@ export const PrefsSchema: z<SidebarPrefs> = z.object({
   htmlViewerDefaultUnsafe: z.boolean().default(false),
   browserNoSandbox: z.boolean().default(false),
   browserInterceptLinks: z.boolean().default(true),
+  browserAllowedLoopback: z.string().default(''),
   // Per-feature enable switches are OPEN maps (any tab/viewer id, built-in or
   // external): an absent key means enabled, so old documents resolve to {}
   // (everything on) with no migration. Non-boolean values fail validation.