chat: refine conversation typography

[0xSero]

Purpose

Make conversation replies materially easier to read without changing session data or behavior.

Changes

• keep prose prominent, tighten mobile markdown leading, and size code separately
• preserve complete code blocks in horizontally scrollable views rather than ellipsizing them
• apply the same code face/scale and line spacing to Android Markwon
• exercise a prose, inline-code, and fenced-code fixture in the iOS display harness

Verification

• xcodebuild test -project apps/ios/Litter.xcodeproj -scheme Litter -destination "platform=iOS Simulator,name=iPhone 17 Pro" -only-testing:LitterUITests/LitterUITests/testConversationDisplayExpandedModeShowsAllDetails -quiet
• JAVA_HOME=/opt/homebrew/opt/openjdk@21/libexec/openjdk.jdk/Contents/Home ANDROID_SDK_ROOT=/opt/homebrew/share/android-commandlinetools ANDROID_NDK_HOME=/opt/homebrew/share/android-commandlinetools/ndk/30.0.14904198 ./gradlew :app:testDebugUnitTest --tests com.litter.android.ui.LitterAppearanceModeTest
• Installed the iOS app on an iPhone 17 Pro Max simulator and captured the conversation fixture; the finished fenced block retains both lines without truncation.

[0xSero]

Superseded by #295 (v2.0.1 mega convergence). Equivalent mega commit: 98981fa8 — chat: refine conversation typography [skip mobile-release] (patch-id 6b05176b, 10/10 blob-verified). Closing; not merging.