fix: global search case-insensitive + keyboard layout swap

Unified search normalization across InternalLinkPicker and GlobalSearch:

1. GlobalSearch.svelte: multi-variant search (same as InternalLinkPicker)
   - expandKeyboardVariants() for RU/EN layout swap
   - Parallel Search queries with dedup by type+nodeId+targetId+title
   - 180ms debounce preserved

2. Backend: fix LOWER() in SQL for links/actions
   - Replace LOWER(column) LIKE with lowercased columns (title_lower, url_lower, etc.)
   - Migration 020: add lowercased columns + indexes for links and actions
   - BackfillLinksLower() + BackfillActionsLower() in storage.go
   - Update INSERT in bindings_links.go and action.go to populate lowercased columns

3. FTS5 search: Unicode case-insensitive
   - Index lowercased title/content/tags in search_index
   - sanitizeFTS() now lowercases query before MATCH
   - RebuildFTS() called after migrations

4. Case-insensitive search for nodes (already done in previous commit, verified):
   - title_lower column with Go strings.ToLower
   - Search() queries title_lower with lowercased query

All test suites PASS, full build OK.

internal/core/search/search.go

@@ -32,7 +32,7 @@ func (s *Service) Index(nodeID, title, content, path, tags, docType string) erro
 	_, err := s.db.Exec(
 		`INSERT INTO search_index (node_id,title,content,path,tags,type)
 		 VALUES (?,?,?,?,?,?)`,
-		nodeID, title, content, path, tags, docType,
+		nodeID, strings.ToLower(title), strings.ToLower(content), path, strings.ToLower(tags), docType,
 	)
 	return err
 }
@@ -92,7 +92,9 @@ func (s *Service) Search(query string) ([]Result, error) {
 
 func sanitizeFTS(q string) string {
 	// Wrap in double quotes for phrase search, escape inner quotes.
+	// Also lowercase the query since we index lowercased content.
 	q = strings.TrimSpace(q)
+	q = strings.ToLower(q)
 	q = strings.ReplaceAll(q, `"`, `""`)
 	return `"` + q + `"`
 }