feat: configure provider defaults for new threads

[asfires]

What Changed

• Added configurable provider defaults for new threads, including model options and runtime mode.
• Applied configured defaults across web and mobile thread creation flows, while preserving explicit draft selections.
• Added settings UI and search support for managing provider-specific new-thread defaults.
• Consolidated fork migrations into the primary migration ledger and simplified development database migration handling.
• Updated development session cookie isolation to use instance-specific hashes.
• Added and updated focused tests and user documentation.

Why

Users can now define provider-specific defaults once and have new threads consistently start with the intended model, options, and runtime mode. Explicit selections continue to take precedence. The migration and cookie changes simplify fork maintenance and improve isolation for parallel development servers.

UI Changes

This changes the provider settings UI and new-thread model selection behavior. Before/after screenshots are not included.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes