fix(tui): disambiguate /model picker rows when provider display names collide

If the gateway returns two providers that resolve to the same display name
(e.g. `kimi-coding` and `kimi-coding-cn` both → "Kimi For Coding"), the
picker now appends the slug so users can tell them apart, in both the
provider list and the selected-provider header. No-op when names are
already unique.

Refs #10526 — the Python backend dedupe from #10599 skips one alias, but
user-defined providers, canonical overlays, and future regressions can
still surface as indistinguishable rows in the picker. This is a
client-side safety net on top of that.

ui-tui/src/domain/providers.ts

@@ -0,0 +1,17 @@
+export const providerDisplayNames = (providers: readonly { name: string; slug: string }[]): string[] => {
+  const counts = new Map<string, number>()
+
+  for (const p of providers) {
+    counts.set(p.name, (counts.get(p.name) ?? 0) + 1)
+  }
+
+  return providers.map(p => {
+    const dup = (counts.get(p.name) ?? 0) > 1
+
+    if (!dup || !p.slug || p.slug === p.name) {
+      return p.name
+    }
+
+    return `${p.name} (${p.slug})`
+  })
+}