ui: Replace non-null assertions on index accesses with checked constructs

Since noUncheckedIndexedAccess was enabled, many index accesses were
silenced with '!'. This removes those assertions in the obvious cases,
following the guidance in AGENTS-ui.md:

- Convert index loops to for...of / arr.entries() where the index is
  only used for indexing (charts, datagrid, DataExplorer, plugins).
- Read-then-check instead of checking length and then indexing (scroll
  details panel, line chart, trace config version check).
- Use ensureDefined() where an invariant genuinely needs asserting.
- Widen sqlValuesEqual() to accept undefined, which it already
  normalised, dropping the assertions in groupPathsEqual().
- Zip key/column pairs once per decode in SliceTrack so the per-row
  loop needs no indexing at all.

Hot paths keep indexed iteration (with a comment explaining why the
index is provably in range): the flamegraph renderer and the per-frame
slice hover-variant loop, where .entries() would allocate a pair per
iteration.

No behaviour changes are intended. ui/build --typecheck, ESLint and
Prettier are clean; all unit tests pass.