* 🪟 fix: Re-measure sidebar chat list on width change to fix date-group spacing
When the sidebar is expanded from a collapsed reload, virtualized rows first
measure mid-animation at a narrow width, so date-group headers wrap and cache an
inflated height. CellMeasurerCache(fixedWidth) keys heights by row, not width, so
the stale height persists once full width is reached — leaving gaps under headers.
Invalidate the measurement cache and recompute row heights whenever the measured
list width changes. Adds a Playwright mock e2e (seeds backdated convos across date
groups via a new db helper) that fails without the fix and passes with it.
* 🧪 test: Harden sidebar e2e (runtime-env path, midnight-safe seed, convo isolation)
Addresses Codex review on PR #13981:
- db.ts honors E2E_RUNTIME_ENV_PATH when locating the runtime Mongo URI.
- Seed timestamps anchor on local noon so the Today group stays in-day near midnight.
- Clear the shared user's conversations before seeding so later date-group headers
are not pushed below the virtualized viewport by other specs' leftover chats.
