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/lib.rs

@@ -22,6 +22,8 @@ pub fn run() {
             commands::write_to_pane,
             commands::resize_pane,
             commands::kill_pane,
+            commands::save_workspace,
+            commands::load_workspace,
         ])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");