SafeBackend: explicit per-cell cursor positioning eliminates ghost glyphs

Terminals and fonts disagree about the rendered width of some glyphs
(emoji like ⭐/➕). Ratatui's stock backend writes contiguous runs and
trusts the terminal to advance exactly unicode-width columns, so a
single disagreement shifts the rest of the run — misplaced characters
then stick around ('ghosts') until something else repaints the cell.

The new SafeBackend positions the cursor explicitly (ESC[y;xH) before
every cell it writes, so width disagreements can never accumulate.
Diffs are small, so the ~8 bytes/cell overhead is negligible. Verified
in tmux: rendering, dragging and IPC unchanged; 41/41 integration
checks pass.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/main.rs

@@ -1,4 +1,5 @@
 mod app;
+mod backend;
 mod client;
 mod config;
 mod ipc;
@@ -17,7 +18,7 @@ use crossterm::{
     terminal::{disable_raw_mode, enable_raw_mode, EnterAlternateScreen, LeaveAlternateScreen},
 };
 use ipc::{ClientMessage, ClientRole, ServerMessage, WindowInfo};
-use ratatui::{backend::CrosstermBackend, Terminal};
+use ratatui::Terminal;
 use server::IpcEvent;
 use std::collections::HashMap;
 use std::io::{self, Write as _};
@@ -220,7 +221,9 @@ fn run_standalone() -> io::Result<()> {
             )
         );
     }
-    let backend = CrosstermBackend::new(stdout);
+    // SafeBackend positionerar varje cell explicit — immun mot terminal/
+    // font-oenighet om glyfbredder (spök-tecken vid fönsterflytt).
+    let backend = backend::SafeBackend::new(stdout);
     let mut terminal = Terminal::new(backend)?;
 
     let config = Config::load_or_default("config.toml");
@@ -239,7 +242,7 @@ fn run_standalone() -> io::Result<()> {
 }
 
 fn run_standalone_loop(
-    terminal: &mut Terminal<CrosstermBackend<io::Stdout>>,
+    terminal: &mut Terminal<backend::SafeBackend<io::Stdout>>,
     app: &mut App,
     ipc_rx: mpsc::Receiver<IpcEvent>,
     _socket_path: &str,
@@ -401,7 +404,7 @@ fn get_terminal_pixel_size() -> (u16, u16) {
 
 /// Uppdatera Kitty-bakgrundsbild om det behövs (ny bild, storleksändring, borttagen).
 fn update_background(
-    terminal: &mut Terminal<CrosstermBackend<io::Stdout>>,
+    terminal: &mut Terminal<backend::SafeBackend<io::Stdout>>,
     app: &App,
     bg_state: &mut Option<kitty_gfx::BgImage>,
     cols: u16,