feat: order Packets oldest-top/newest-bottom with up/down row navigation

[sg]

Summary

Two Packets-screen changes:

1. Oldest at top, newest at the bottom (reverse of before). New packets append to the bottom and older rows scroll up — like a log/terminal tail.
2. Up/Down arrow keys move to the previous/next row, selecting it and showing it in the Inspector.

Changes (web-ui/src/views/Packets.vue)

• Initial load reverses the API's newest-first list and scrolls to the bottom.
• WS packets now push to the bottom (was unshift); the 500-cap trims the oldest via shift (was pop).
• Auto-tail: a new packet scrolls to the bottom only if the user is already at the bottom, so it won't yank focus while they're scrolled up reading an older packet.
• Keyboard nav: a keydown listener (mounted only for this view) — Up = previous (older/above), Down = next (newer/below). Selects the row, loads it in the Inspector, and scrollIntoView({block:'nearest'}). Ignores arrows when focus is in input/select/textarea or with Ctrl/Cmd/Alt; preventDefaults page scroll. With nothing selected, the first arrow selects the newest row.
• Rebuilt webapi/static/ assets.

Testing

Front-end only. npm run build clean (no JS test harness in the repo).