fix gui input clamping, dropdown selector, pip dedup and other bugs

[nghiphaam]

A batch of GUI fixes found while reviewing the web app:

• domDropdown.js: the doesDropdownItemExist selector was missing its closing ], so querySelector threw a SyntaxError when saving/reading any dropdown setting (engine, weight, etc.).
• domInputs.js: range clamping used ... || min, which coerced a legitimately-clamped 0 to min, and if(min && max) skipped clamping when min === 0. Now clamps with a null check and only falls back to min on NaN.
• gui.js: GET_TTS_VOICES() returns false when unsupported, which fell through to .forEach and threw every 100ms; now guarded with !ttsVoices?.length. Also added a null check before closeBtn.onclick.
• externalEngine.js: the "previously selected engine not available" branch (else if(!foundSelectedEngineID ...)) was unreachable; restructured so it actually warns. Engine-id comparisons now use String() consistently.
• pip.js: renderPipBoards() was called without from/to, so the new-suggestion dedup never matched; now passes them and uses ||. Eval fallback now treats a real 0 as valid and falls back to the last eval instead of leaving it undefined (which produced NaN).
• settings.js: in the no-instance + profile branch, base was read before config[type] was initialized, so INIT_NESTED_OBJECT could throw; it's now initialized first.
• dynamicEngineOptions.js: the timeout cleanup filtered by w.resolve !== resolve, which never matched the stored wrapper, leaking timed-out waiters; now filters by waiter identity.
• instances.js: guarded .highlight-indicator handlers against a missing element.