fix(server): preprocess oversized images before sending to agent SDK

[caoer]

Summary

• The Claude Agent SDK resizes images exceeding 2000×2000px using sharp internally, but sharp may fail to load in the spawned CLI process, producing a [System Error] Unable to resize image — dimensions exceed the 2000x2000px limit and image processing failed error
• Adds a preprocessImages() step in the daemon's session layer that resizes oversized images before dispatching to any agent provider — sharp is available in the daemon process from the hoisted SDK dependency
• Gracefully falls back to sending the original image if sharp is unavailable, so this is purely additive

Changes

• packages/server/src/server/agent/preprocess-image.ts — New utility: dynamically imports sharp, checks image dimensions, resizes to fit within 2000×2000 (preserving aspect ratio) and re-encodes as JPEG quality 80
• packages/server/src/server/agent/preprocess-image.test.ts — Tests: small images pass through unchanged, oversized images get resized with correct aspect ratio, corrupt data handled gracefully
• packages/server/src/server/session.ts — Both handleSendAgentMessage() and the send_agent_message_request handler now call preprocessImages() before buildAgentPrompt()

Test plan

• All 7 unit tests pass (small image passthrough, oversized resize, aspect ratio preservation, corrupt data fallback, undefined/empty array handling, multiple images)
• Typecheck passes across all packages
• Lint + format clean
• Manual: attach a >2000px image from the app and verify it reaches the agent without [System Error]