PROMPT UPLOAD

# Prompt Upload & Version Advancement

> **Phase**: 1.5 — implement before Phase 2 (Evaluation Engine)
>
> **Purpose**: Allow users to load prompt files into the comparison UI via drag-and-drop or file picker, have them automatically persisted to the backend, and support advancing the prompt pair (B becomes A, new B loaded) to represent the next iteration of the comparison session.

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## Overview

The current UI requires pasting prompt text manually. This feature adds:

1. **Drag-and-drop** on either PromptPanel to load a `.txt` or `.md` file
2. **Browse file button** as an alternative to drag-and-drop
3. **Subtle upload indicator** while the file is being read and saved to the backend
4. **"Use as Prompt A →" button** on the Prompt B panel to advance the session to the next iteration

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## Drag-and-Drop & File Upload

### Behavior

1. User drags a `.txt` or `.md` file over either PromptPanel editor.
2. The panel highlights (border pulse) while the file is dragged over it.
3. On drop, the file is read via `FileReader.readAsText(file)`.
4. The textarea is **immediately populated** with the file content — no waiting for a network call.
5. In parallel, the content is POSTed to the backend:
   - If no prompt manifest exists for this panel slot: `POST /api/eval/prompts` (creates a new prompt)
   - If a prompt manifest already exists (file was previously loaded): `POST /api/eval/prompts/:id/versions` (adds a new version)
6. A subtle status strip below the textarea shows: `Reading file…` → `Saved` → (clears after 2s) or `Error saving` (persists until dismissed).

### Supported file types

- `.md` — Markdown system prompts
- `.txt` — Plain text prompts
- Unsupported types: show inline error `"Only .md and .txt files are supported"`; do not clear textarea.

### Browse File Button

A small "Browse…" link/button sits below the textarea alongside the drag hint text.
Clicking it triggers a hidden `<input type="file" accept=".txt,.md">`.
Same behavior as drop — reads file, populates textarea, saves to backend.

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## PromptPanel Component Changes

**File: `src/components/prompt/PromptPanel.tsx`**

### New props