## Symptom
The colored byte strip in the packet detail pane is offset from the
labeled byte breakdown below it. Off by N bytes where N is the
difference between the top-level packet's path length and the displayed
observation's path length.
## Root cause
Server computes `breakdown.ranges` once from the top-level packet's
raw_hex (in `BuildBreakdown`) and ships it in the API response. After
#882 we render each observation's own raw_hex, but we keep using the
top-level breakdown — so a 7-hop top-level packet shipped "Path: bytes
2-8", and when we rendered an 8-hop observation we coloured 7 of the 8
path bytes and bled into the payload.
The labeled rows below (which use `buildFieldTable`) parse the displayed
raw_hex on the client, so they were correct — they just didn't match the
strip above.
## Fix
Port `BuildBreakdown()` to JS as `computeBreakdownRanges()` in `app.js`.
Use it in `renderDetail()` from the actually-rendered (per-obs) raw_hex.
## Test
Manually verified the JS function output matches the Go implementation
for FLOOD/non-transport, transport, ADVERT, and direct-advert (zero
hops) cases.
Closes nothing (caught in post-tag bug bash).
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Co-authored-by: you <you@example.com>
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