fix: activity back navigation + preserve reel scroll on sheet close

[GraysonCAdams]

Summary

• Tapping a notification in /activity opened the clip via `goto('/?clip=X')`, which pushed the feed entry and then the overlay's own entry. One back press only popped the overlay, leaving the user on the feed instead of returning to /activity. The activity link is now tagged with `from=activity`; the overlay reads it and on dismiss pops the extra feed entry so the user lands directly back on /activity.
• MeReelView's mount effect re-ran whenever a nested sheet (e.g. CommentsSheet) pushed/popped state, and the `tick()` scroll-to-startIndex call fired each time — snapping the reel back to the originally-tapped clip. Guard the initial scroll with a one-shot flag so only the first mount positions the reel.

Test plan

• Tap an activity notification → clip opens → tap back arrow (or swipe right) → lands on /activity, not the feed
• From Me → Uploads, tap a clip several rows down in the grid → scroll down through several clips in the reel → open comments → close comments → reel stays at the clip you were viewing (not back to the originally tapped one)
• Normal feed: open comments on a clip mid-list → close comments → feed stays on that clip