Theseus: image context menu (open / save / copy)

Right-click on an image now offers:
- Open image in new tab   -> createTab(srcURL)
- Save image as...        -> wc.downloadURL(srcURL). The tab's
                             will-download handler leaves savePath
                             unset, so Electron shows the native
                             Save As dialog and the download appears
                             in the tracker.
- Copy image              -> wc.copyImageAt(x, y). Puts the decoded
                             image on the clipboard (paste into any
                             image-aware target).
- Copy image address      -> clipboard.writeText(srcURL).

Only shown when p.mediaType === "image" && p.srcURL, so text /
link right-clicks are unchanged.

main.js

@@ -1299,6 +1299,18 @@ function createTab(initial, opts = {}) {
         { type: "separator" },
       );
     }
+    // Image context menu: only when the pointer is actually on an image, and
+    // we have a src to act on. Save-image-as triggers will-download with no
+    // preset savePath, so Electron shows the native Save As dialog.
+    if (p.mediaType === "image" && p.srcURL) {
+      items.push(
+        { label: "Open image in new tab", click: () => createTab(p.srcURL) },
+        { label: "Save image as…",        click: () => wc.downloadURL(p.srcURL) },
+        { label: "Copy image",            click: () => { try { wc.copyImageAt(p.x, p.y); } catch {} } },
+        { label: "Copy image address",    click: () => clipboard.writeText(p.srcURL) },
+        { type: "separator" },
+      );
+    }
     if (p.isEditable) items.push({ role: "cut" }, { role: "copy" }, { role: "paste" }, { type: "separator" });
     else if (p.selectionText) items.push({ role: "copy" }, { type: "separator" });
     items.push(