feat: packet payload hex+ASCII viewer (#46)

[NotYuSheng]

Summary

• Extracts raw packet bytes during PCAP parsing and stores the first 1024 bytes as a lowercase hex string per packet
• Conversation detail panel now opens as a full modal-xl with prev/next navigation and keyboard shortcuts
• Clicking a packet row in the stream expands an inline hex+ASCII dump (Wireshark-style); rows with significant cleartext are flagged with an ASCII badge

Changes

Backend

• V12__add_payload_to_packets.sql — new migration
• PcapParserService — extractPayloadHex() helper converts raw frame bytes to hex (capped at 1024 bytes); threaded through processIpPacket → buildPacketInfo; tshark fallback leaves payload null
• PacketEntity, PacketResponse, AnalysisService — payload field propagated end-to-end

Frontend

• HexViewer.tsx — new component rendering offset / hex (8+8 split) / ASCII columns in a dark monospace panel with a truncation notice
• ConversationDetail — clickable packet rows expand HexViewer; yellow ASCII badge on rows where >30% of bytes are printable ASCII; hint text in card header
• ConversationPage — split-panel replaced with modal-xl; ←/→ arrow keys navigate prev/next; ESC or backdrop click closes; X/N counter in header; hint text on conversations card

Test plan

• Upload ftp.pcap — verify packet rows show ASCII badge and hex dump displays readable FTP commands/responses on click
• Verify encrypted traffic (TLS) shows unreadable bytes with no ASCII badge
• Test ESC, ←/→ arrow keys, backdrop click, and Prev/Next buttons in the modal
• Confirm DB migration V12 runs cleanly
• Verify packets parsed via tshark fallback still load (payload will be empty)

Closes #46

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request introduces the ability to view packet payloads in a hex viewer within the conversation detail view. Changes include backend updates to store and serve the first 1024 bytes of packet data, a new database migration, and a frontend refactor that moves conversation details into a modal with keyboard navigation. Feedback focuses on improving performance in hex string conversion, fixing potential stale state issues in React hooks, addressing magic numbers and hardcoded styles, and ensuring proper error logging and UI alignment.