Fix active-pane detection via activeElement polling

After exhausting event-based approaches that all failed in WebView2:
- per-leaf onpointerdown: xterm.js stopPropagation
- document-capture pointerdown: only first event ever delivered
- document-capture mousedown/click: never delivered at all
- document-capture focusin: silently fails
- term.onFocus: no such xterm.js API

The bulletproof fallback: poll document.activeElement every 250ms
and call orch.setActive on its closest [data-leaf-id] ancestor.
No DOM events involved. Verified working with automation: clicking
pane 2 turns its border blue, clicking pane 1 moves the border to
pane 1, etc.

XtermPane gained an onFocus prop (still wired through LeafPane) as a
secondary signal that might fire in some configurations, but the
polling is the actual fix.

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

src/components/XtermPane.svelte

@@ -20,6 +20,7 @@
     onSpawn = undefined,
     onInput = undefined,
     onDataReceived = undefined,
+    onFocus = undefined,
     focusTrigger = 0,
   }: {
     distro?: string;
@@ -31,6 +32,8 @@
     onInput?: (dataB64: string) => void;
     /** Fired whenever output arrives from the PTY. Used for idle detection. */
     onDataReceived?: () => void;
+    /** Fired when xterm's textarea gains focus (i.e., user clicked here). */
+    onFocus?: () => void;
     /** Increment to refocus the terminal programmatically (palette etc.). */
     focusTrigger?: number;
   } = $props();
@@ -112,6 +115,20 @@
       onInput?.(b64);
     });
 
+    // xterm.js's own focus event — fires when the hidden textarea gets focus
+    // (i.e., user clicked anywhere in the terminal). Most reliable signal
+    // for "user wants this pane active" — no DOM event traversal involved.
+    term.onSelectionChange(() => {}); // ensure the addon system is initialized; noop
+    if (typeof (term as unknown as { onFocus?: unknown }).onFocus === "function") {
+      (term as unknown as { onFocus: (cb: () => void) => void }).onFocus(() => {
+        onFocus?.();
+      });
+    } else {
+      // Fallback: listen on the textarea element directly.
+      const ta = containerEl.querySelector(".xterm-helper-textarea");
+      if (ta) ta.addEventListener("focus", () => onFocus?.(), true);
+    }
+
     // Re-fit on container resize; forward new size to the PTY.
     ro = new ResizeObserver(() => {
       try {