Persistent in-list day separator and aligned bubble grouping

[kerdx]

Chat timeline improvements following motd's MessageList design (Telegram-style grouping + day dividers).

• New persistent day chip (DaySeparatorRow reusing DateChip: Oggi/Ieri/date) drawn inside the row that opens a new day — never a separate item, so indexes, keys and jump math don't shift. Shown on message and presence-burst rows, in both Bubbles and IRC-line modes
• Shared pure helpers in ChatTimeline.kt: isSameDay() + continuesMessageGroup() (same sender, 5-min window kept, breaks on system/chat change, /me ACTION boundary, or day change)
• tight grouping now uses the shared predicate (plus the existing unread-divider guard)

Tests: 4 new unit tests in ChatTimelineTest (14 total, all passing). Verified: testDebugUnitTest green, assembleDebug builds.