fix(traces): stop the cold-start render loop in the trace store

[htcom-code]

What & why

Every cold start logged Maximum update depth exceeded ~50 times before any
data arrived. App passed data?.traces ?? [] into useTraceStore — the ??
fallback builds a new array on every render, and the hook's sample-mode
effect (src/hooks/use-trace-store.ts:52) lists that array in its dependencies
and calls setRows with rows derived from it. So: effect → setRows → render →
new [] → effect, until React's nested-update cap cut it off. It self-resolved
once the first snapshot (or the sample fallback at 2.5s) gave data.traces a
stable identity, which is why the UI looked fine — it just burned ~50 render
passes and filled the console at each start.

Two changes:

• Root cause — App.tsx hands over a module-level NO_TRACES constant, so
  "no traces yet" has one stable identity.
• Prevention — the sample-mode effect now commits passthrough only when
  the row keys actually differ (sameRows), so a future caller with an unstable
  array cannot reopen the loop. Nothing in the render path can catch this class
  of bug — tsc and oxlint both pass on the broken version.

Live mode is untouched: that effect returns early when persist is true.

Type of change

• fix — bug fix (no new behaviour)

Correctness & conventions

• HTTP stays in the data layer only (src/lib/api.ts); components take plain props.
• src/lib/types.ts unchanged (no backend surface change).
• The LIVE / SAMPLE-DATA fallback still works (UI explorable with no platform).
• No visual change.

Verification

• npm run lint passes (3 pre-existing only-export-components warnings, unchanged)
• npm run build passes (tsc -b && vite build)
• Verified in the app (npm run dev, SAMPLE data, no platform on 8080):
  console is clean apart from the expected 502 from the absent stream —
  Maximum update depth exceeded went from ~50 occurrences per load to 0
• Clear-history still works on the sample path (the flow this change touches):
  arm + confirm empties the table to 0 shown · 0 retained, the "No traces yet"
  empty state renders, and the button disables
• LIVE mode not exercised — no platform on :8080 in this environment. The
  changed effect is skipped entirely when persist is true, so the live
  seed/merge path is unaffected by construction.

Baseline for the counts above was an unmodified main worktree, which showed the
same error at 53 occurrences before this change.