Fix whitespace-only mappings highlighting every space on the page

A mapping imported with real: " " (single space) passed the !m.real
guard (space is truthy), producing new RegExp(" ", "gi") which matched
every space character in the DOM and highlighted them all.

- Added .trim() to all real/substitute guard conditions in content.js
  (inline substitute, reveal functions) and substitution-engine.js so
  whitespace-only values are treated as empty and skipped
- Added the same guard in highlightMatches so the CSS Highlight API
  never creates ranges for blank search terms
- Added trim + blank-check to the bulk import path in options.js so
  whitespace-only real values are dropped at import time rather than
  saved to storage

https://claude.ai/code/session_01CwcZK8nqL8pyBH9AxDs9qo

src/content/content.js

@@ -50,7 +50,7 @@
     const sorted = [...maps].sort((a, b) => b.real.length - a.real.length);
 
     for (const m of sorted) {
-      if (!m.enabled || !m.real || !m.substitute) continue;
+      if (!m.enabled || !m.real?.trim() || !m.substitute?.trim()) continue;
       const escaped = esc(m.real);
       const regex = new RegExp(escaped, m.caseSensitive ? 'g' : 'gi');
       let match;
@@ -70,7 +70,7 @@
     let result = text;
     const sorted = [...maps].sort((a, b) => b.substitute.length - a.substitute.length);
     for (const m of sorted) {
-      if (!m.enabled || !m.real || !m.substitute) continue;
+      if (!m.enabled || !m.real?.trim() || !m.substitute?.trim()) continue;
       const escaped = esc(m.substitute);
       const regex = new RegExp(escaped, m.caseSensitive ? 'g' : 'gi');
       result = result.replace(regex, m.real);
@@ -1130,6 +1130,7 @@
 
       for (const p of pairs) {
         const searchTerm = settings.revealMode ? p.to : p.from;
+        if (!searchTerm?.trim()) continue;
         const escaped = esc(searchTerm);
         // Add word boundaries when the term starts/ends with word chars to
         // prevent partial-word matches (e.g. "aud" inside "Claude")