Add M3: APPDATA persistence + presets + per-pane distro/label

Backend:
- save_workspace / load_workspace Tauri commands writing to
  %APPDATA%\com.megaproxy.tiletopia\workspace.json with atomic
  tmp+rename. Path from app.path().app_config_dir() (no dirs crate).

Layout helpers:
- tree.ts: changeDistro (with id swap to force XtermPane remount via
  {#key}), changeLabel, presetSingle / TwoColumns / ThreeColumns /
  TwoRows / TwoByTwo.
- New ops.ts with PaneOps interface bundling split / close /
  setDistro / setLabel / distros, drilled through Pane chain
  instead of individual callbacks.

UI:
- LeafPane: in-toolbar editable label (click to rename, Enter
  saves, Esc cancels) and distro chip popover. Picking a different
  distro respawns the pane.
- App.svelte: migrated from localStorage to APPDATA via the new
  Tauri commands, debounced 500ms. One-time localStorage migration
  on boot. Split inherits parent's distro+cwd. Titlebar preset
  buttons with confirm when replacing >1 pane.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src-tauri/src/commands.rs

@@ -1,10 +1,12 @@
 //! Tauri command surface. Every JS-callable function lives here.
 
 use base64::{engine::general_purpose::STANDARD as B64, Engine as _};
-use tauri::AppHandle;
+use tauri::{AppHandle, Manager};
 
 use crate::pty::{list_wsl_distros, PaneId, PtyManager};
 
+const WORKSPACE_FILE: &str = "workspace.json";
+
 #[tauri::command]
 pub async fn list_distros() -> Result<Vec<String>, String> {
     list_wsl_distros().map_err(|e| e.to_string())
@@ -55,3 +57,38 @@ pub async fn kill_pane(
 ) -> Result<(), String> {
     manager.kill(id).map_err(|e| e.to_string())
 }
+
+/// Write the workspace JSON to `%APPDATA%\com.megaproxy.tiletopia\workspace.json`.
+/// Writes to a `.tmp` and renames over the real file so a crash mid-write
+/// can't leave a partial file readable.
+#[tauri::command]
+pub async fn save_workspace(app: AppHandle, json: String) -> Result<(), String> {
+    let dir = app
+        .path()
+        .app_config_dir()
+        .map_err(|e| format!("app_config_dir: {e}"))?;
+    std::fs::create_dir_all(&dir).map_err(|e| format!("create_dir_all: {e}"))?;
+    let path = dir.join(WORKSPACE_FILE);
+    let tmp = dir.join(format!("{WORKSPACE_FILE}.tmp"));
+    std::fs::write(&tmp, json.as_bytes()).map_err(|e| format!("write tmp: {e}"))?;
+    // `std::fs::rename` is atomic on Unix and uses MoveFileEx with
+    // REPLACE_EXISTING on Windows (>= Rust 1.50).
+    std::fs::rename(&tmp, &path).map_err(|e| format!("rename: {e}"))?;
+    Ok(())
+}
+
+/// Read the workspace JSON. Returns `None` if the file doesn't exist yet
+/// (first launch).
+#[tauri::command]
+pub async fn load_workspace(app: AppHandle) -> Result<Option<String>, String> {
+    let dir = app
+        .path()
+        .app_config_dir()
+        .map_err(|e| format!("app_config_dir: {e}"))?;
+    let path = dir.join(WORKSPACE_FILE);
+    if !path.exists() {
+        return Ok(None);
+    }
+    let s = std::fs::read_to_string(&path).map_err(|e| format!("read: {e}"))?;
+    Ok(Some(s))
+}