fix: Resolve web package lint and typecheck errors

Fixes ESLint and TypeScript errors in web package to pass CI checks:

- Fixed all Quality Rails violations (14 explicit any types)
- Fixed deprecated React event types (FormEvent → SyntheticEvent)
- Fixed 26 TypeScript errors (Promise types, test mocks, HTMLElement assertions)
- Added vitest DOM matcher type definitions
- Fixed unused variables and empty functions
- Resolved 43+ additional lint errors

Typecheck: ✅ 0 errors
Lint: 542 remaining (non-blocking in CI)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/web/src/components/knowledge/LinkAutocomplete.tsx

@@ -8,7 +8,7 @@ interface LinkAutocompleteProps {
   /**
    * The textarea element to attach autocomplete to
    */
-  textareaRef: React.RefObject<HTMLTextAreaElement>;
+  textareaRef: React.RefObject<HTMLTextAreaElement | null>;
   /**
    * Callback when a link is selected
    */
@@ -82,7 +82,7 @@ export function LinkAutocomplete({
 
       setResults(searchResults);
       setSelectedIndex(0);
-    } catch (_error) {
+    } catch (error) {
       console.error("Failed to search entries:", error);
       setResults([]);
     } finally {
@@ -116,7 +116,7 @@ export function LinkAutocomplete({
       const styles = window.getComputedStyle(textarea);
 
       // Copy relevant styles
-      [
+      const stylesToCopy = [
         "fontFamily",
         "fontSize",
         "fontWeight",
@@ -127,10 +127,13 @@ export function LinkAutocomplete({
         "boxSizing",
         "whiteSpace",
         "wordWrap",
-      ].forEach((prop) => {
-        mirror.style[prop as keyof CSSStyleDeclaration] = styles[
-          prop as keyof CSSStyleDeclaration
-        ] as string;
+      ] as const;
+
+      stylesToCopy.forEach((prop) => {
+        const value = styles.getPropertyValue(prop);
+        if (value) {
+          mirror.style.setProperty(prop, value);
+        }
       });
 
       mirror.style.position = "absolute";