fix(#583): add Google (Gemini) to model provider selection

The model-config provider dropdown in the Providers settings screen
(also shown in the in-chat Providers dialog) only offered Custom,
OpenRouter, Anthropic, and OpenAI. Despite docker-compose exposing
GOOGLE_API_KEY, users could not select Google/Gemini as a provider
without hand-editing config. Add 'google' to ModelProviderOption and
MODEL_PROVIDER_OPTIONS (label 'Google (Gemini)'), and register 'google'
as a known provider prefix so google/* model ids strip correctly.

src/screens/settings/providers-screen.tsx

@@ -57,6 +57,7 @@ const KNOWN_PROVIDER_PREFIXES = [
   'anthropic',
   'openai',
   'openai-codex',
+  'google',
   'nous',
   'ollama',
   'atomic-chat',
@@ -767,7 +768,12 @@ function SettingCard(props: {
   )
 }
 
-type ModelProviderOption = 'custom' | 'openrouter' | 'anthropic' | 'openai'
+type ModelProviderOption =
+  | 'custom'
+  | 'openrouter'
+  | 'anthropic'
+  | 'openai'
+  | 'google'
 
 type ModelConfigDraft = {
   provider: ModelProviderOption
@@ -785,6 +791,7 @@ const MODEL_PROVIDER_OPTIONS: Array<SelectOption> = [
   { label: 'OpenRouter', value: 'openrouter' },
   { label: 'Anthropic', value: 'anthropic' },
   { label: 'OpenAI', value: 'openai' },
+  { label: 'Google (Gemini)', value: 'google' },
 ]
 
 const MODEL_PRESETS = [