Typing indicator: continuous dot animation, safe visibility fallback, layout branch by controller

[byaletheya-sudo]

Four small defects in typing_indicator.dart, all visible from the code:

• Periodic hitch. The dots looped by listening for AnimationStatus.completed and calling forward(from: 0.0) from the status callback, which stops and restarts the Ticker; the first tick after a restart reports zero elapsed time, so every dot froze for one frame every 700 ms. AnimationController.repeat() runs the identical 0..pi tween on one continuous ticker, and the status listener (which could call forward() on a controller being torn down) goes away.
• Null assertion. (widget.controller?.showTypingIndicatorFor.isNotEmpty ?? widget.visible)! throws if a caller passes neither argument; now ?? false.
• Wrong layout branch in tablet mode. The branch tested iOS || cm.activeChat == null, but the only call sites without a controller are the chat-list tiles, and activeChat is non-null whenever a chat is open in the other pane. On a wide screen the tiles took the Material branch, where participants is an empty list, and drew an empty avatar group beside the dots. Branching on widget.controller == null is the intended distinction.
• Icon overflow. The app-icon container in the ClipPath branch had no height while its Material sibling has height: 25; with 10/20 padding in a 50 px row, an undecorated Image.memory overflowed. Set to 20. AnimatedSize is anchored to the leading edge so the dots no longer slide sideways while it resizes.

Verification. One file. Built as a release APK (arm64) from this branch and installed on a Galaxy Z Fold 8; the app runs. I could not observe a live typing indicator in a sideloaded build (registration needs the hosted service), so the fixes are argued from the code above; the periodic hitch is reproducible by reasoning about Ticker restart and should be visible in any 1:1 chat while the other side types. flutter analyze reports no new diagnostics against rustpush.

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