refactor(tui): deduplicate cursor_cell_offset into text_width module

Four view modules (search, command_palette, file_history, trace) each had
their own copy of cursor_cell_offset / text_cell_width for converting a
byte-offset cursor position to a display-column offset. Phase 5 introduced
a proper text_width module with these functions; this commit removes the
duplicates and rewires all call sites to use crate::text_width.

- search.rs: removed local text_cell_width + cursor_cell_offset definitions
- command_palette.rs: removed local cursor_cell_offset definition
- file_history.rs: replaced inline chars().count() cursor calc with import
- trace.rs: replaced inline chars().count() cursor calc with import

crates/lore-tui/src/view/file_history.rs

@@ -21,8 +21,9 @@ use ftui::render::cell::{Cell, PackedRgba};
 use ftui::render::drawing::Draw;
 use ftui::render::frame::Frame;
 
-use crate::state::file_history::{FileHistoryResult, FileHistoryState};
 use super::common::truncate_str;
+use crate::state::file_history::{FileHistoryResult, FileHistoryState};
+use crate::text_width::cursor_cell_offset;
 
 // ---------------------------------------------------------------------------
 // Colors (Flexoki palette)
@@ -137,8 +138,7 @@ fn render_path_input(frame: &mut Frame<'_>, state: &FileHistoryState, x: u16, y:
 
     // Cursor indicator.
     if state.path_focused {
-        let cursor_col = state.path_input[..state.path_cursor].chars().count() as u16;
-        let cursor_x = after_label + cursor_col;
+        let cursor_x = after_label + cursor_cell_offset(&state.path_input, state.path_cursor);
         if cursor_x < max_x {
             let cursor_cell = Cell {
                 fg: PackedRgba::rgb(0x10, 0x0F, 0x0F), // dark bg