feat: allow configuring custom gateway endpoint and token from TUI settings

[suryaiyer95]

Problem

Setting up a custom LLM gateway (e.g. corporate Bedrock proxy) requires editing JSON config files and shell profiles outside of Altimate Code. There is no way to configure the gateway endpoint or bearer token from within the TUI itself.

Proposal

Add a TUI settings flow (accessible from the provider/model configuration UI) that lets users:

1. Set a custom gateway URL for any provider — writes provider.<id>.options.baseURL to the global config file.
2. Enter a bearer token inline — stores it via the existing Auth.set() mechanism (same as providers login on the CLI).

This removes the need to manually create ~/.config/altimate-code/altimate-code.json or set environment variables for first-time gateway setup.

Relevant Code

• Provider handler: packages/opencode/src/provider/provider.ts:301 ("amazon-bedrock" block)
• Config schema: packages/opencode/src/config/config.ts:1008 (provider options)
• CLI login flow: packages/opencode/src/cli/cmd/providers.ts:250 (ProvidersLoginCommand)
• Auth storage: packages/opencode/src/auth/service.ts
• Global config write: packages/opencode/src/config/config.ts:1532 (globalConfigFile())