[Feature Request] Custom API Headers & Body Parameters per Model

## The Problem

Anthropic just released Claude Opus 4.6 Fast Mode — a research preview that delivers up to 2.5x faster output token generation. To use it, you need to pass two things that BoltAI currently doesn’t support:

1. A custom HTTP header: anthropic-beta: fast-mode-2026-02-01

1. A custom body parameter: "speed": "fast"

The model ID stays the same (`claude-opus-4-6`), so you can’t work around this by just adding a new model. Without these fields, the request goes through as standard Opus 4.6 — no speed boost, and no way to opt in.

This isn’t just about Fast Mode. Anthropic (and other providers) increasingly use beta headers and extra body parameters for new features:

- anthropic-beta: context-1m-2025-08-07 — 1M token context window

- anthropic-beta: prompt-caching-2024-07-31 — prompt caching

- speed: "fast" — fast inference mode

- inference_geo: "us" — data residency controls

Right now, none of these are accessible from BoltAI.

## The Suggestion

In the Edit Model screen, add two optional fields:

- Custom Headers — a key-value input (or raw text field) for extra HTTP headers.

- Custom Body Parameters — a key-value input (or JSON field) for additional parameters injected into the request body.

### Example Configuration

For Opus 4.6 Fast Mode:

|Field |Value |

|-----------------|--------------------------------------|

|Model ID |`claude-opus-4-6` |

|Custom Header |`anthropic-beta: fast-mode-2026-02-01`|

|Custom Body Param|`"speed": "fast"` |

For 1M context window:

|Field |Value |

|-------------|---------------------------------------|

|Model ID |`claude-opus-4-6` |

|Custom Header|`anthropic-beta: context-1m-2025-08-07`|

## Why It Fits BoltAI

BoltAI already gives power users granular control over model parameters like Top-P, Temperature, and Frequency Penalty. Custom headers and body params are the natural next step — they unlock every beta/preview feature from any provider without waiting for BoltAI to add explicit UI support for each one.

This is a forward-looking solution: instead of playing catch-up with every new API flag, you give users the tools to configure it themselves on day one.

## Priority

High — Fast Mode is available now and providers are shipping new beta features at an accelerating pace. Every week without this means users either can’t access new capabilities or have to fall back to curl/scripts.