feat: add interactive provider selection

[acidsugarx]

Problem or motivation

Add ability to interactively select translation provider before translating, instead of using the one from config.

Proposed solution

Add a new command/keymap that opens a picker (telescope/fzf/snacks/mini/native) with available providers:

• Google Translate
• DeepL (if API key configured)
• (future providers)
  After selecting a provider, proceed with translation using that provider for the current request only.

1. New command: :BabelSelect — select provider, then translate selected text
2. New keymap: <leader>ts (translate with selector)
3. Use existing picker system from ui.lua

Alternatives considered

No response

Additional context

Questions

• Should the selection be remembered for the session, or only for single translation?
• Should unavailable providers (e.g., DeepL without API key) be shown but disabled, or hidden?
• Should this also allow selecting target language?

Related

#2 — DeepL provider

Checklist

• I have searched existing issues to ensure this feature has not already been requested.