feat(ai): OpenRouter provider and AI settings section

[SobshDev]

Summary

• Adds an OpenRouterService implementing AiService (chat, streaming with tool calls, structured output) and wires it into createAiService.
• Expands the chat model catalog with three OpenRouter-backed models: DeepSeek Chat v3.1, Grok 4.3, GPT-5.4 Nano.
• Adds an AI section to Settings with a per-provider key row (save, reveal/hide, remove); the chat provider exposes a new removeKey action.

Test plan

• Open Settings → AI and confirm three provider rows render (Claude, Codex, OpenRouter).
• Save, reveal, then remove an OpenRouter key; reload and confirm persistence/clearing.
• Pick a DeepSeek/Grok/GPT model in the chat model picker and send a message — verify text streams in.
• Trigger a tool call from an OpenRouter model and verify the round-trip completes (tool result fed back, final message returned).
• Confirm Claude and Codex flows still work unchanged.