Fix 6 bugs: tab hijacking, cache expiry, NaN settings, debugger error handling

[Copilot]

Six independent bugs in background.js and popup.js ranging from crashes to silent data corruption.

background.js

• getTabId() crash + tab hijacking: tabs[0] was accessed unconditionally — undefined when no active tab exists (auto-start alarm). Now falls back to chrome.tabs.create({ active: false }) to avoid hijacking the user's current tab and to prevent the TypeError.

• onConnect popup-close handler: On popup disconnect, code re-queried the current active tab to detach the debugger — wrong tab entirely. Now uses searchState.tabId directly; no-ops if null.

• trendingWordsCache never expired: Cached value was a bare array with no timestamp, surviving the full service worker lifetime regardless of TRENDS_CACHE_DURATION. Now stored as { terms, timestamp } with expiry checked on every read.

• Unhandled chrome.runtime.lastError: enableDebugger/disableDebugger didn't read lastError in their callbacks, producing unchecked-error console noise when a tab was closed mid-search. Both now handle it.

• randomDelay() with inverted min/max: If user sets max < min, max - min goes negative. Fixed with Math.min/Math.max normalization:

  const lo = Math.min(min, max);
  const hi = Math.max(min, max);
  return Math.floor(Math.random() * (hi - lo + 1) + lo);

popup.js

• NaN from empty form fields: parseInt("") returns NaN, which propagates into totalSearches and breaks progress math. Added || default fallbacks on every parseInt() call in both sendAutoStartSettingsToBackground() and startSearches().