feat(dashboard): packet detail modal in live feed

[iceice400]

Summary

• Replace inline JSON row expand in the live packet feed with a structured Packet detail modal
• Layers: RF (freq, modem, RSSI, SNR), Mesh (from/to, hops, channel hash, relay byte), Payload (decrypt success/failure + content), Capture (packet id, source, timestamp)
• Close via ✕, Escape, or backdrop; feed keeps updating behind the modal; selected row highlight; map focus via existing setOnFocus

Why

Operators need a quick layered breakdown of each packet without scrolling raw JSON in the table. This is PR 1 of the operator diagnostics suite (frontend only, no new API).

Type

• Bug fix
• Feature
• Docs
• Refactor
• UI
• Installer
• Region support
• Hardware change

Testing

• Local only (code review / static)
• Tested on hardware
• Dashboard tested on device

Manual checklist (on gateway):

1. Open dashboard → Live feed tab; click a packet row → modal opens with four layers
2. Encrypted / no-key packet shows Decrypt: No matching key (red)
3. Decrypted text/position/telemetry shows human-readable Content
4. Close with ✕, Escape, and backdrop click; row highlight clears
5. While modal open, new packets still append to feed
6. Clicking a row still focuses source node on map (if map tab wired)

Hardware: RAK7248 or Pi + HAT · US915 (recommended before merge)

Impact

• Parsing
• Relay
• TX
• Radio driver
• Region logic
• UI only
• Installer only

Risks: Low — frontend-only; reads existing WebSocket packet.to_dict() payload. Deferred: multi-channel duplicate breakdown, channel name from hash map, ToA estimate.

AI-assisted?

• Yes