fix(replay): hide duration while chunks are progressively loading

[ayushjhanwar-png]

Problem

When opening a session replay, the duration display jumps around as chunks load progressively:

• T=0: `2:11 / 2:57` (first batch only — 50 chunks)
• T=2s: `2:11 / 5:30` (after second batch fetched)
• T=5s: `2:11 / 14:58` (final, all chunks in)

The growing duration looks like a bug — users assume the recording is shorter than it is and try to scrub, only to have the timeline expand under them. Reported during 30% rollout monitoring.

Root cause

`ReplayPlayer` initializes with the first 50-chunk batch and shows `getMetaData().totalTime` from that subset. `ReplayChunkLoader` then recursively fetches remaining batches in the background, calls `addEvent()` for each event, and triggers `refreshDuration()` after each batch. `refreshDuration()` re-reads `getMetaData().totalTime` and updates state — so the displayed duration visibly climbs as more events arrive.

Fix

Track a `chunksLoaded: boolean` in `ReplayContext`:

• `ReplayChunkLoader` sets it true when its recursive fetch finishes (`hasMore === false` OR error)
• A new `ReplayChunksLoadedFlag` component covers the edge case where the first batch already contains everything (loader never mounts) — sets the flag immediately
• `ReplayTime` shows `…` while `chunksLoaded` is false, then renders the final duration in one step

The duration is still computed and refreshed the same way underneath; we just hide the in-progress value from the user.

Files

• `replay-context.tsx` — added `chunksLoaded` state + setter, reset on destroy
• `index.tsx` — `ReplayChunkLoader` flips the flag on completion; new `ReplayChunksLoadedFlag` for single-batch sessions
• `replay-controls.tsx` — `ReplayTime` shows `…` while loading

Test plan

• Open a long session (e.g. `2a8a077c-...` with 7,583 chunks): duration shows `…` while loading, then jumps to ~7+ hours once everything is in
• Open a short session (`<50` chunks): duration appears immediately without flicker
• Open a session with a load failure mid-stream: duration eventually appears (we set the flag in `.catch` too)