feat: add prefill progress bar for long prompts

[AlexCheema]

Motivation

Users processing long prompts have no visibility into when token generation will start. This feature adds a progress bar showing prefill progress, giving users real-time feedback during prompt processing.

Changes

Backend

• Added PrefillProgress event type with command_id, processed_tokens, total_tokens
• Added PrefillProgressResponse type (though now using direct callback approach)
• Wired prompt_progress_callback through MLX's stream_generate()
• Progress events sent directly from callback for real-time updates (not batched)
• API generates SSE named events: event: prefill_progress\ndata: {...}
• Added PrefillProgressData dataclass and StreamEvent union type in API

Dashboard

• Added PrefillProgress interface to store
• Updated SSE parsing to handle event: lines (named events)
• Created PrefillProgressBar.svelte with animated progress bar
• Shows "Processing prompt: X/Y tokens" with percentage
• Progress bar disappears when first token arrives

Why It Works

MLX's stream_generate() accepts a prompt_progress_callback(processed, total) that's called after each prefill chunk. By sending events directly from this callback (rather than yielding from the generator), progress updates are sent in real-time during prefill.

Using SSE named events (event: prefill_progress) maintains full OpenAI/Claude API compatibility - standard clients ignore named events they don't recognize, while the exo dashboard explicitly listens for them.

Test Plan

Manual Testing

• Hardware: MacBook Pro M3 Max
• Set prefill_step_size=256 for more frequent updates
• Tested with long prompts (pasted large documents)
• Verified progress bar updates incrementally during prefill
• Confirmed progress bar disappears when generation starts
• Tested with curl - standard data: events still work normally

Automated Testing

• Type checker passes (0 errors)
• All 192 tests pass
• Dashboard builds successfully

API Compatibility

• Named SSE events are ignored by OpenAI SDK clients
• Regular token data uses standard data: {...} format
• [DONE] sentinel works as expected

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Note: prefill_step_size is temporarily set to 256 for testing. Should be changed back to 2048 before merging for production performance.