fix(app): composer textarea shrinks back down on web

Measuring scrollHeight on the live textarea with an explicit height
set can only grow — it can never report a shorter content height.
The old `height: auto` toggle flickered across browsers.

Use a hidden off-DOM textarea that mirrors the composer's text-metric
styles and width, and read its scrollHeight on value/resize changes.
Gated by Metro file extension (.web / .native) so it's trivial to
rip out later.

packages/app/src/components/composer-height-mirror.d.ts

@@ -0,0 +1 @@
+export * from "./composer-height-mirror.native";