[Feature Request] Support dynamic variables in Custom Headers for per-conversation Session IDs

Description: Currently, Page Assist supports adding Custom Headers in the API provider settings (e.g., OpenAI Compatible / Advanced Options). However, these headers are static and applied globally across all chats.

The Problem: API providers like OpenCode Go require a unique session header per conversation (e.g., x-opencode-session) to optimize server-side caching and route requests properly (vendor enforcing from 2026-09-06). Their strict requirement is "one stable ID per conversation."

Because Page Assist only allows hardcoded custom headers, using a fixed value (e.g., x-opencode-session: my-fixed-id) causes every new chat in the extension to send the exact same Session ID.

Proposed Solution: Introduce a dynamic variable system that can be parsed within the Custom Headers input field.

For example, allowing a placeholder like {{conversation_id}} or {{chat_uuid}} that automatically injects the internal ID of the current active chat thread.

Example configuration in the Custom Headers UI:

"x-opencode-session": "{{conversation_id}}"

Expected Behavior:

• When a user creates a New Chat, a new unique ID is generated and sent in the header.
• When a user sends follow-up messages in the same chat, the header remains stable and sends the same ID.

This small addition would make Page Assist fully compatible with API caching endpoints without requiring hardcoded integrations for every specific provider.