ui: two-column layout for conversation pane with marker gutter

Split conversation pane into 2-char gutter + text column. Gutter shows
● markers at turn boundaries (Cyan for user, Magenta for assistant),
aligned with the input area's ' > ' gutter.

Key changes:
- Added Marker enum (None/User/Assistant) and parallel markers vec
- Track turn boundaries via pending_marker field
- New draw_conversation_pane() with visual row computation for wrapping
- Both gutter and text scroll synchronously by visual line offset

This fixes the wrapping alignment issue where continuation lines
aligned under markers instead of under the text.

Cargo.lock

@@ -2804,6 +2804,7 @@ dependencies = [
  "tiktoken-rs",
  "tokio",
  "tui-markdown",
+ "tui-textarea-2",
  "unicode-width",
  "walkdir",
 ]
@@ -4601,6 +4602,20 @@ dependencies = [
  "tracing",
 ]
 
+[[package]]
+name = "tui-textarea-2"
+version = "0.10.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "74a31ca0965e3ff6a7ac5ecb02b20a88b4f68ebf138d8ae438e8510b27a1f00f"
+dependencies = [
+ "crossterm 0.29.0",
+ "portable-atomic",
+ "ratatui-core",
+ "ratatui-widgets",
+ "unicode-segmentation",
+ "unicode-width",
+]
+
 [[package]]
 name = "typenum"
 version = "1.19.0"