Raise the browser the extension reported, and add a diagnostics script

focus.py raised the first browser in its list that happened to be running. On a
machine with both Chrome and Edge installed that is a coin flip, and losing it is
silent: coordinates measured from a tab in one browser, the click delivered into
a window of the other. It presents as selectors failing for no reason. A VPS with
both installed hit exactly this.

The extension now reports which browser is hosting it, and that travels with the
measurement, so the clicker raises the browser the coordinates actually came
from. Asking for a browser that is not running now fails honestly instead of
quietly raising a different one, and the verification step rejects the wrong
browser coming forward. Chromium, Opera and Vivaldi are recognised alongside
Chrome, Edge and Brave, on both platforms.

diagnose.py answers the question a remote desktop makes hard: whether the mouse
is really moving or the viewer simply is not drawing it. It moves the cursor and
reads the position back from the OS, so the answer does not depend on anything
being rendered, and it reports DPI mode, screen size, whether this is an RDP
session, and whether the browser can be raised at all. Nothing is clicked.

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

extension/background.js

@@ -154,6 +154,24 @@ async function captureAndSend(cfg) {
     });
 }
 
+/** Which browser is hosting this extension.
+ *
+ *  The clicker has to raise *this* browser before clicking, and picking by list
+ *  order gets it wrong the moment two are installed — it would raise Edge while
+ *  the coordinates came from a tab in Chrome, landing every click in the wrong
+ *  window. So the answer travels with the measurement.
+ */
+function detectBrowser() {
+    const ua = navigator.userAgent || '';
+    if (/\bEdg\//.test(ua)) return 'edge';
+    if (/\bOPR\//.test(ua)) return 'opera';
+    if (/\bVivaldi\//.test(ua)) return 'vivaldi';
+    try {
+        if (navigator.brave) return 'brave';   // Brave otherwise reports as Chrome
+    } catch { /* not Brave */ }
+    return 'chrome';
+}
+
 // ── Locate: turn a CSS selector into desktop coordinates ────────────────────
 
 /** Runs in the page. Scrolls the element into view, then reports where it ended up. */
@@ -322,7 +340,13 @@ async function serveLocateRequest(cfg) {
             if (Date.now() >= deadline) throw new Error(out.error);
             await new Promise((r) => setTimeout(r, 350));
         }
-        result = { ...out, tabId: tab.id, windowId: tab.windowId, openedTab: opened };
+        result = {
+            ...out,
+            tabId: tab.id,
+            windowId: tab.windowId,
+            openedTab: opened,
+            browser: detectBrowser(),
+        };
     } catch (err) {
         error = err.message;
     }