feat: add browserdata/datautil helpers (#513)

* feat: add browserdata/datautil helpers (QuerySQLite, QueryRows, DecryptChromiumValue)

Phase 2 of architecture refactoring (RFC-002 Section 3):

- datautil/sqlite.go: QuerySQLite() — shared SQLite open/query/scan helper
  with optional journal_mode=off for Firefox databases
- datautil/query.go: QueryRows[T]() — generic helper (Go 1.20) that wraps
  QuerySQLite and collects results into a typed slice
- datautil/decrypt.go: DecryptChromiumValue() — unified Chromium decryption
  (DPAPI first, then AES-GCM/CBC fallback)
- datautil/sqlite_test.go: tests for all helpers

* refactor: move DecryptChromiumValue from datautil to browser/chromium

- Remove browserdata/datautil/decrypt.go (Chromium-specific, not a generic util)
- Will be added as browser/chromium/decrypt.go (unexported decryptValue)
  in the chromium extract methods PR
- Update RFCs to reflect the change
- Remove decrypt test from datautil tests

* refactor: move datautil to utils/sqliteutil for consistency

- Rename browserdata/datautil/ → utils/sqliteutil/
- Aligns with existing utils/ convention (fileutil, typeutil, byteutil)
- QuerySQLite/QueryRows are generic SQLite helpers, not browserdata-specific
- Update package name from datautil to sqliteutil
- Update both RFCs to reflect new location

* fix: apply review suggestions for sqliteutil

- QuerySQLite: validate dbPath exists before sql.Open to prevent
  silently creating empty databases
- Tests: check db.Close() errors with require.NoError

utils/sqliteutil/query.go

@@ -0,0 +1,21 @@
+package sqliteutil
+
+import "database/sql"
+
+// QueryRows is a generic helper (Go 1.18+) that wraps QuerySQLite and collects
+// results into a typed slice. Each extract method only needs to provide the
+// scan function that converts one database row into a typed value.
+//
+// Rows that fail to scan are skipped (logged at debug level by QuerySQLite).
+func QueryRows[T any](dbPath string, journalOff bool, query string, scanRow func(*sql.Rows) (T, error)) ([]T, error) {
+	var items []T
+	err := QuerySQLite(dbPath, journalOff, query, func(rows *sql.Rows) error {
+		item, err := scanRow(rows)
+		if err != nil {
+			return err
+		}
+		items = append(items, item)
+		return nil
+	})
+	return items, err
+}