fix(test): restore green Jest suite (jsdom 29 pin + snek coverage) (#382)

Two independent regressions kept "Run JavaScript Tests" red on main and
blocked unrelated dependabot PRs:

1. jest-environment-jsdom was bumped to ^30.3.0 (jsdom 26) while jest core
   stayed at 29. jsdom 26 makes window.location non-configurable, so the
   `delete window.location` / Object.defineProperty mocks used across the
   frontend tests throw ("Cannot redefine property: location" /
   "navigation not implemented"), failing 11 tests. Pin
   jest-environment-jsdom back to ^29.7.0 to match jest core and restore
   jsdom 20 location semantics. Verified: full suite green on jsdom 20.

2. snek.js coverage fell below threshold after #340 added the newsletter
   speech bubble without tests (functions 28.57% vs 40% required). Add a
   "Newsletter Speech Bubble" test block covering dismissSpeechBubble, both
   dismiss handlers, and the dismissed/not-dismissed branches. snek.js now
   exceeds all thresholds (stmts/lines 90%, branches 100%, funcs 71%) with
   no threshold relaxation.

Full suite: 15 suites, 423 tests passing with --coverage --ci.

Follow-up (separate PR): modernise the location mocks (history.replaceState)
so the suite can move to jest + jsdom v30.

Author: JesperDramsch