test(desktop): unit tests for main.rs's decisions and a browser test for the connect page

main.rs was the only source file without tests, and it held three decisions
with real failure modes:

- Which login a connect goes out with. The choice was a closure body inside
  the probe worker; it is now choose_login, tested for every row: typed wins,
  then the Keychain, then the local Codeman's own login, which is used only
  when no saved password was expected and only for the same username.
- What the Keychain keeps afterwards (update_keychain): saved or deleted by
  Remember, never a copy of the local server's login, never a password the
  server wants changed, and the old user's item deleted before the new save.
  The in-memory Keychain store moves out of credentials' test module so other
  test modules can use it.
- When a commit in the main window means the server page is loaded.

The connect page had only substring checks in the repository; the checks that
drove it lived in a scratch script. They are now
test/desktop-connect-page.browser.test.ts: Chromium, the page's real CSP, a
stubbed native side, five ordered walks through every state of both tabs
(first run, logins, failures, redirects, Start, the install wait, the SSH
install, layout, double clicks, keys, a starting server, a WebKit too old).
Listed in BROWSER_TEST_GLOBS, so the nightly browser suite runs it and
`npm test` does not.

Also pins the install one-liner the page shows to the one Copy copies.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

config/test-suites.ts

@@ -60,6 +60,7 @@ export const BROWSER_TEST_GLOBS = [
   'test/fullscreen-tab-switch-capture.browser.test.ts',
   'test/terminal-touch-swipe-axis.browser.test.ts',
   'test/phone-press-feedback.browser.test.ts',
+  'test/desktop-connect-page.browser.test.ts', // no server: the page is served by request interception
 ];
 
 /**