feat: add multi-account support for auth providers

[coleleavitt]

Summary

• Add support for multiple accounts per provider with active account switching
• Automatic migration from legacy single-auth format

Changes

Auth Module (src/auth/index.ts)

• New ProviderAuth data structure with accounts map and active field
• New functions: listAccounts, setAccount, setActive, removeAccount, getAccount, hasAccounts, getActive
• Backward-compatible migration: legacy single-auth entries auto-convert to multi-account format on read

CLI (src/cli/cmd/auth.ts)

• auth list now shows accounts grouped by provider with active indicator
• auth login accepts --account / -a flag to specify account name
• auth logout allows removing specific accounts (not just entire providers)

TUI (src/cli/cmd/tui/component/dialog-provider.tsx)

• New DialogAccounts component for listing/switching accounts
• New DialogAccountName component for naming new accounts
• Updated DialogAuthMethod to support account context

Server (src/server/server.ts)

• New endpoints:
  • GET /auth/:providerID/accounts - List accounts
  • PUT /auth/:providerID/accounts/:accountName - Set account credentials
  • POST /auth/:providerID/accounts/:accountName/active - Set active account
  • DELETE /auth/:providerID/accounts/:accountName - Remove account

SDK

• Regenerated with new auth.accounts.* methods