fix(desktop): memoize sidebar flatRows and row renderers (#75714 salvage)

[kshitijk4poor]

Salvage of #75714 by @StanleyStetson — both commits cherry-picked to preserve authorship (rebased clean), plus one lint follow-up commit.

Context — what this changes for users

Sidebar scrolling jittered because every render handed VirtualSessionList a FRESH flatRows array — same data, new identity — re-rendering the whole virtualized tree for nothing (the desktop guide's "preserve reference identity on no-ops" invariant). Memoizing flatRows (and the row renderers) keeps the reference stable across unrelated re-renders; the PR's second commit adds tests that pin exactly this (identity stable across re-render, changed when sessions actually change — verified regression-sensitive: removing the memo or either dep fails them).

Review & verification (real vitest runs)

• Dep-array audit: all 5 memo/callback arrays complete against their captured variables (flatRows: [dateGrouped, displayEntries]; renderRow: all 9 captured handlers/values — no stale-handler risk).
• Comparator interplay traced: SidebarSessionRow's rowPropsEqual deliberately ignores callback identity, so nothing defeats it; the flatRows useMemo is the piece the tests prove and the perf fix users feel. The useCallback wrappers are inert-but-harmless groundwork (their consumers aren't memo()'d yet) — noted, not blocking.
• 2/2 tests green, tsc clean; exported VirtualSessionListProps matches the codebase's established *Props export convention (~27 precedents).

Follow-up commit

• eslint --fix on the new test file (perfectionist/sort-imports would have failed lint CI).

Closes #75714.