fix(trajectory): display sub-second durations as exact milliseconds

formatElapsedSeconds rounded every duration to one decimal, so calls
under 50 ms rendered as 0 s. Durations below one second now show integer
milliseconds (29 ms); at or above one second the tenth-of-a-second label
is unchanged (1.5 s). Step group descriptions reuse the same formatter.

The details-panel Duration rows toggle between the readable label and
exact milliseconds on click, mirroring the StartedAt toggle; the shared
text-selection guard is extracted for both. tabular-nums is dropped from
detail values and the toggle buttons because SF's tnum feature widens
the decimal point and leaves excessive space after it.

packages/client/ui-trajectory/tests/cell.spec.tsx

@@ -20,7 +20,10 @@ describe('formatElapsedSeconds', () => {
     expect(formatElapsedSeconds(235.0)).toBe('235 s')
     expect(formatElapsedSeconds(235.2)).toBe('235.2 s')
     expect(formatElapsedSeconds(235.25)).toBe('235.3 s')
-    expect(formatElapsedSeconds(0)).toBe('0 s')
+    expect(formatElapsedSeconds(0)).toBe('0 ms')
+    expect(formatElapsedSeconds(0.029)).toBe('29 ms')
+    expect(formatElapsedSeconds(0.5)).toBe('500 ms')
+    expect(formatElapsedSeconds(1.5)).toBe('1.5 s')
     expect(formatElapsedSeconds(Number.NaN)).toBe('—')
   })
 })