feat(servers): allow custom display names for NNTP providers

[javi11]

Summary

• Adds an optional Name field to server/provider configuration so users can label each NNTP provider (e.g. "Astraweb Primary") instead of the auto-numbered "Provider 1, 2, 3..." shown in the setup wizard, settings page, and dashboard/metrics.
• Verified against the current nntppool library (pinned v4.11.1, checked up through the latest v4.13.0/main): it has no Name field on Provider and only auto-derives an internal name from host+username for its own logs/stats. This is therefore a Postie-side config/UI feature only — no dependency bump, and the custom name is never sent to the connection pool.
• Falls back to the existing "Provider N"/host-based display exactly as before when no name is set, so existing configs are unaffected.

Closes #220

Changes

• Backend: ServerConfig.Name (internal/config/config.go), ServerData.Name (internal/backend/app.go), NntpProviderMetrics.Name; wired through SetupWizardComplete and GetNntpPoolMetrics's host→config lookup.
• Frontend: new "Name" input + header fallback in NntpServerManager.svelte (shared by setup wizard and settings); Wails bindings (models.ts); dashboard/metrics display now shows provider.name || provider.host; translations added for en/es/fr/tr.
• Bug fix found during testing: ServerSection.svelte (Settings page) and ServerSetupStep.svelte (setup wizard) both rebuild server objects through explicit field whitelists that silently dropped any field not listed. Added name to each of these whitelists — otherwise a typed name would render once but be stripped on the next save.
• Minor docs touch: config-example.yaml, docs/docs/configuration.md.

Test plan

• go build ./internal/... and go vet ./internal/...
• go test ./internal/config/... (existing suite, no regressions)
• bun run check (svelte-check) — 0 errors/warnings
• Manually verified end-to-end in a sandboxed environment (isolated $HOME, real Go web server + Vite dev): typed a name in the setup wizard with live header update, confirmed round-trip through YAML → JSON config API, persisted through a Settings-page save, and confirmed it renders on the /metrics page's provider table in place of the raw host.
• Manual smoke test in the packaged Wails desktop app (not run in this environment)