Bug Report

Problem 1: Model list overflow in terminal selection menus

When using a custom provider that returns a large number of models (e.g., an OpenAI-compatible endpoint with 100+ models), the selectOne and selectMany menus in the setup wizard render all items at once. If the model list exceeds the terminal window height:

• Items beyond the visible area are never shown
• Arrow key navigation only cycles through the visible items
• Users cannot access or select models that overflow the terminal

This affects all call sites in internal/cli/cli.go that use selectOne/selectMany:

• selectEnabledProviders() — provider family and model selection
• promptCustomProviderFromURL() — custom provider model selection
• promptAnthropicProviderFromURL() — Anthropic provider model selection

Problem 2: No /provider command to switch providers in CLI

When entering the CLI chat interface, there is no obvious way to switch between configured providers. The /model command requires knowing the full provider/model reference. Users need a /provider command to list and switch providers more intuitively.

Proposed Solution

Fix 1: Viewport scrolling + keyword search in select menus

• Add terminal height detection via term.GetSize()
• Implement viewport windowing that shows only N items that fit in the terminal
• Add scroll indicators ("↑ N more above" / "↓ N more below")
• Add '/' key to enter search/filter mode for keyword-based filtering

Fix 2: /provider slash command

• /provider — list all configured providers with model counts
• /provider <name> — switch to that provider's default model
• Tab autocomplete for provider names
• Bilingual i18n support (en/zh)

Files Affected

• internal/cli/select.go — viewport scrolling + search
• internal/cli/provider.go — new /provider command
• internal/cli/chat_tui.go — register /provider
• internal/cli/complete.go — /provider autocomplete
• internal/cli/help_view.go — /provider in help
• internal/cli/model.go — providerNames() helper
• internal/control/slash.go — /provider arg completion
• internal/i18n/i18n.go — new message fields
• internal/i18n/messages_en.go — English translations
• internal/i18n/messages_zh.go — Chinese translations