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

## 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..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()`)