fix(mobile): polish channel management and iOS Settings (#8171)

## Summary

Use native iOS sheets for channel editing and Add Members, with profile
avatars, matching search, and destructive removal dialogs. Canvas
content becomes a tappable preview with Add/Remove actions.

Fix the Settings availability menu and clipped copy confirmation. Align
channel action messages with chat styling and keep agent identities
distinguishable across selection, attribution, and profile surfaces.

### Related issue

No duplicate found in the existing mobile Settings/channel issues and
PRs.

### Testing

- Flutter analysis, formatting, file-size checks, focused collision
regressions, and the full mobile test suite passed.
- Production UIKit picker simulator check passed: distinct
visible/accessibility labels, avatar updates, selected rows, and pubkey
callbacks. Wired into Mobile Swift CI.
- Original UI was built and tested on the physical iPhone with author
confirmation; profile/community data preserved.
- Review fixes await renewed author device confirmation before restoring
`buzz-review-completed` and ready-for-review status.

---------

Signed-off-by: kenny lopez <klopez4212@gmail.com>

scripts/test-mobile-member-picker.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+# Exercise the production UIKit member picker 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 "$fixture"' EXIT
+xcrun --sdk iphonesimulator swiftc \
+  -sdk "$(xcrun --sdk iphonesimulator --show-sdk-path)" \
+  -target "$(uname -m)-apple-ios16.0-simulator" \
+  -F "$frameworks" -framework Flutter \
+  -Xlinker -rpath -Xlinker "$frameworks" \
+  "$repo_root/mobile/ios/Runner/NativeAddMembersSheet.swift" \
+  "$repo_root/mobile/ios/NativeControlTests/AddMembersIdentityTests.swift" \
+  -o "$fixture/member-picker-tests"
+xcrun simctl spawn "$device" "$fixture/member-picker-tests"