fix: align DayAnimated sticky header with inline separators (Telegram-style push)

[kesha-antonov]

Problem

The animated day header (DayAnimated) is a floating overlay, while the day separators are rendered inline in the list. For the effect to read as a single iOS-Telegram-style sticky header, the two must hand off at the exact same screen line. They didn't:

• The floating header pins at DAY_PIN_OFFSET, but an inline separator's pill sits DAY_MARGIN_TOP lower (the Day container's marginTop), while the floating header overrides that margin to 0.
• The handoff used the wrong threshold, so the inline separator stayed fully visible until it had scrolled well past the floating header.

Result, visible at every day boundary while scrolling: a brief duplicate date badge (same date shown twice) and a vertical jump as the date switched.

Fix

Introduce a shared DAY_HANDOFF_OFFSET = DAY_PIN_OFFSET - DAY_MARGIN_TOP used by both sides so they hand off at the same pixel:

• Floating header picks the stuck day at separatorScreenTop <= DAY_HANDOFF_OFFSET, and the next (newer) day's separator pushes it up: top = min(DAY_PIN_OFFSET, nextSeparatorScreenTop + DAY_MARGIN_TOP - headerHeight), so the floating pill's bottom rests exactly on the rising separator's pill. The outgoing date slides up and off as the incoming arrives - a single clean push.
• Inline separator fades out across [DAY_HANDOFF_OFFSET, DAY_HANDOFF_OFFSET + fade], i.e. exactly as it reaches the line where the floating header takes over. No duplicate, no gap, no jump.

Fade timing tightened to match Telegram (fast fade-in, short idle delay, then fade-out), behind a DEBUG_TIME_SCALE constant (=1) for future frame-by-frame tuning. Also dropped the now-unused messages prop from DayAnimated.

Validation

Validated on the Android simulator with a 7-day data set, stepping through day boundaries frame-by-frame:

• Floating header shows the correct top day, pinned.
• At a boundary the outgoing date is pushed up and off while the incoming arrives at the same line - no duplicate badge, no jump.
• At rest the header fades out (Telegram shows the date only while scrolling).