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/java/com/m57/hermescontrol/theme/ChatStyle.kt

@@ -1,11 +0,0 @@
-package com.m57.hermescontrol.theme
-
-/**
- * Chat message rendering style (issue #866). App-local preference,
- * persisted in [com.m57.hermescontrol.data.config.ServerStoreState].
- *
- * BUBBLES — legacy bubble renderer (default, unchanged behavior).
- * FULL_BLEED — agent prose renders directly on the background with turn
- *              headers; user messages keep bubbles (parallel renderer).
- */
-enum class ChatStyle { BUBBLES, FULL_BLEED }