feat(chat): remove legacy bubble renderer, full-bleed is the only chat style (#881)

Deletes the ChatMessageList/ChatStreaming bubble path and the ChatStyle
setting (issue #866 phase 6). ChatScreen now renders FullBleedChatList
directly; ChatBubble becomes the user-message anchor with the shared
SystemBubble/InlineAttachment/formatTimestamp helpers intact. Stubs of
AuthManager.getChatStyle() removed from four test suites; TodoTaskCardTest
renders ToolBubble directly.

Verified: ktlintCheck green, testDebugUnitTest 962/962 pass, assembleDebug
APK builds.

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/res/values/strings.xml

@@ -163,10 +163,6 @@
     <string name="settings_sec_chat">Chat</string>
     <string name="settings_item_typing_effect">Typing Effect</string>
     <string name="settings_desc_typing_effect">Animate new messages word by word</string>
-    <string name="settings_item_chat_style">Chat style</string>
-    <string name="settings_desc_chat_style">How agent replies are rendered</string>
-    <string name="chat_style_bubbles">Bubbles</string>
-    <string name="chat_style_full_bleed">Full-bleed</string>
     <string name="fullbleed_role_agent">Assistant</string>
     <string name="settings_item_typing_delay">Delay: %1$d ms</string>
     <string name="settings_delay_10ms">10 ms</string>