Modernize mobile channel and DM creation sheets (#8942)

Modernizes New Channel and New DM with full-height forms and X/Create or
X/checkmark actions. iOS uses UIKit sheets throughout; Android uses the
Flutter fallback. Channel menus default to Ongoing/Public, and native
menu previews lift only the trailing value while labels stay fixed.

Channel names disable automatic capitalization, autocorrect, and
spellcheck; both platforms use descriptive placeholders.

Preserves recipient search and limits, creation error retries, and
navigation after native dismissal.

Validation: 4,154 mobile tests passed (38 skipped), auxiliary suites and
static checks passed; iOS device/simulator and Android builds passed.
Exercised menus, dismissal, DM search and selection in the simulator and
Android form behavior on Pixel; iPhone UI tested and approved by the
author.

| iOS native form (Simulator) | Android Flutter form (Pixel) |
| --- | --- |
| ![Native iOS channel
form](https://github.com/user-attachments/assets/a3ee78c9-510d-488c-979e-64fa3bded686)
| ![Android channel
form](https://github.com/user-attachments/assets/a2d65f1c-752c-4cef-b659-2b396cbbbcc9)
|

Native menu with the left label fixed:

![Native value-only menu
preview](https://github.com/user-attachments/assets/c4fe8659-e46c-409f-b866-c9568b26ccc1)

---------

Signed-off-by: kenny lopez <klopez4212@gmail.com>
Co-authored-by: Terp <5856f6900e8e0a4e0ea1f4f8d2ee33e4753d2119bc122f94ecafc660b46c0511@buzz.block.builderlab.xyz>

scripts/test-mobile-create-channel-accessibility.sh

@@ -0,0 +1,21 @@
+#!/bin/bash
+# Exercise the production UIKit create-channel field labels on a booted iOS simulator.
+set -euo pipefail
+repo_root=$(cd "$(dirname "$0")/.." && pwd)
+device=${1:?Pass a booted iOS simulator UDID}
+flutter_root=$(sed -n 's/^FLUTTER_ROOT=//p' "$repo_root/mobile/ios/Flutter/Generated.xcconfig")
+frameworks="$flutter_root/bin/cache/artifacts/engine/ios/Flutter.xcframework/ios-arm64_x86_64-simulator"
+fixture=$(mktemp -d)
+trap 'rm -r "$fixture"' EXIT
+# Compile in one file to exercise the private production controller directly.
+cat "$repo_root/mobile/ios/Runner/NavigationTopEdge.swift" \
+  "$repo_root/mobile/ios/Runner/NativeCreateChannelSheet.swift" \
+  "$repo_root/mobile/ios/NativeControlTests/CreateChannelAccessibilityTests.swift" > "$fixture/test.swift"
+xcrun --sdk iphonesimulator swiftc -parse-as-library \
+  -sdk "$(xcrun --sdk iphonesimulator --show-sdk-path)" \
+  -target "$(uname -m)-apple-ios16.0-simulator" \
+  -F "$frameworks" -framework Flutter \
+  -Xlinker -rpath -Xlinker "$frameworks" \
+  "$fixture/test.swift" \
+  -o "$fixture/create-channel-tests"
+xcrun simctl spawn "$device" "$fixture/create-channel-tests"