[ui] B2 storage: use pipx for the CORS CLI install + rename secret field to "Application Key"

- CORS setup helper: the Backblaze B2 instructions now say `pipx install b2` instead of
  `pip install b2` (the recommended way to install a CLI tool).
- Storage form: for the Backblaze B2 provider the "Secret Access Key" field is now
  labelled "Application Key" to match B2's own terminology (the secret half of an
  Application Key). Other providers keep "Secret Access Key".

Test updated to assert the pipx instruction. Build + lint clean.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

js/ui/profileModal/ProfileStorageController.js

@@ -437,6 +437,13 @@ export class ProfileStorageController {
       this.storageRegionInput.placeholder = "auto";
     }
 
+    // B2 calls the secret an "Application Key" — match its terminology.
+    const secretLabel =
+      this.storageSecretKeyInput?.parentElement?.querySelector("span");
+    if (secretLabel) {
+      secretLabel.textContent = isB2 ? "Application Key" : "Secret Access Key";
+    }
+
     if (this.storageR2Helper) {
       this.storageR2Helper.classList.toggle("hidden", !isR2);
     }